I’m watching Troy:Fall of a City on Netflix. They have cast black actors to play Achilles and his Myrmidon Warriors. It’s a cool way to differentiate them from the other Greeks, but is there any historical reference for this?

There's a lot to unpack here! Correct me if I'm wrong, but it seems to me that your question can only arise from a set of connected assumptions that looks something like this:

The Iliad describes a historical event.
This historical event involved people of a known, homogenous ethnicity.
That ethnicity is best reflected by casting white actors.
Special justification is needed to cast actors who are not white.

They are interconnected in funny ways that reveal a lot about the way we look at the Trojan War cycle. It's worthwhile to think a little more about whether such assumptions make sense.

The key assumption here is the first one. You ask whether there is any historical evidence to back up the casting of black people as Myrmidons, but is there any historical evidence to back up the existence of Myrmidons? Or of any of the characters or events of the Trojan War? The answer is no, unless we indulge the fallacy that any evidence for a conflict in roughly the right area involving roughly the right peoples must be evidence of the Trojan War. In effect, we have nothing concrete to show that the Trojan War really happened, and certainly nothing to attest to the historicity of any of the characters of the epic. The story is just that: a story. It may be set in a real historical context, but not one that its composers understood at any level, and not one that they even attempted to describe accurately. Archaeology has revealed that the epics involve only a bizarre mishmash of Bronze Age arcana placed in a universe that resolutely and coherently reflects the world of Early Archaic Greece. We can discuss the historical context in which the story developed, but we cannot discuss the events of the Homeric epics as if they were real. I wrote about this in more detail here.

This has important implications for modern retellings such as Troy: Fall of a City. If the epics do not make any pretenses to historical accuracy, are modern writers obliged to do better? If we wish to make the setting as authentic as possible, does this mean we should research the Mycenaean age (c. 1500-1100 BC), or the Archaic period (c. 800-500 BC), or the Hellenistic period (323-31 BC), which produced the version of the story that has come down to us? Indeed, if the story is not a history, does it matter where or when it is set? If even the original composer of the epic simply used the Mycenaeans as a vehicle to express the concerns of his own day, is it important to respect its setting, or are we free to "update" the story by placing it in a more relatable or more expressive context, like many modern directors do with the plays of Shakespeare?

There are many possible answers to these questions; every new retelling of the Homeric epics represent its author's perspective. The main point is that it is legitimate to ask whether the Trojan War cycle "belongs" to anyone, and if so, whether or not it is essential that the creators of modern interpretations reflect that ownership.

Many modern commentators take it entirely for granted that the Trojan War cycle belongs to the Greeks (usually meaning both ancient and modern Greeks), and, by extension, that it belongs to the wider Western civilization that is held to have been built on the legacy of the Classical Greeks. Their criticism of Troy: Fall of a City is essentially that the casting of black actors is a poor reflection of the original context and intention of the epics. It supposedly forces a variety of human ethnicities upon the product of a historical society that (again supposedly) would not have been familiar with that variety. In their view, modern retellings of the epics have an obligation to represent its heroes with actors from the societies to which the epics "belong" - namely, Greece and the West.

There are several problems with this line of criticism. First, as I argued above, the notion of the epics as the product of a historical society - requiring all retellings to reflect that society accurately - is deeply problematic. Are we referring to the Mycenaean civilization here, or the Early Archaic one, or an even later and more literate Greek society? If we insist on any one of these to the exclusion of the others, are we not similarly imposing our modern notion of authenticity upon the source material? Can it ever be argued that the epics "belong" to a particular time or place?

Of course, it might be argued that Mycenaeans and Archaic Greeks and Classical Greeks are all Greek, and so the lack of cultural context for the Homeric epics should not be overstated. Ignoring matters of periodization, the story is Greek. But the Iliad itself only mentions "Hellenes" once, having been composed in a time before the Greeks had any collective ethnic consciousness. In the strictest sense, the Homeric epics predate the existence of "the Greeks" as such. Moreover, the epics includes non-Greek characters from Troy and Thrace and Phrygia and Lycia; the wider Trojan War cycle involves characters like Penthesileia the Amazon and Memnon of Aethiopia. This reflects the wider world in which the story was set, which was never about Greeks alone. Is it right to assume that it is a "Greek" story and that we must respect its "Greek" context? It would be easy to point out that the Greeks from the Archaic period onwards would have been familiar with many non-Greek peoples, including peoples from sub-Saharan Africa, whom they incorporated in their stories; Memnon of Aethiopia was typically presented as a black man. But this is missing the point. We don’t need to prove that black people existed in historical Ancient Greek society (though they did), since the story of the Trojan War is neither historical nor exclusively Greek.

It is even more odd to assume that the Trojan War cycle “belongs” to the West, and that the perceived Western inheritors of Greek civilization are somehow uniquely appropriate interpreters of the ancient material whose casting is inherently respectful to the original source. Why should modern white British or American actors be so uniquely capable of doing justice to the epics? Again, it would be easy to counter the critics by pointing out that if they want the closest thing to ethnic authenticity, they really ought to insist that modern Greek actors play all Greek roles in the story, that a modern Ethiopian plays Memnon, and so on. But this, again, misses the point. No one now speaks the languages or lives the experiences of Greeks and other peoples from any of the periods that are intertwined in the epics. No one has greater claim to being “like” the characters as envisioned by the composers of the source. The cultural heritage of Ancient Greece was not directly passed on in an unbroken line anywhere; stories like the Iliad cannot be shown to have had a single owner passing on their priceless possession through the centuries. So why should a retelling performed by a set of British actors be considered more authentic and respectful than a retelling by, say, a group of Senegalese or Cambodian actors?

In short, the real problem that many critics of Troy: Fall of a City raise has nothing to do with the source or its historical context. Neither of these exclude the possibility of diverse casting any more than they exclude the use of English to tell the story. Indeed, we could argue that a diverse cast makes this latest retelling more faithful to its source.

The real problem is best illustrated by one simple fact. In Troy: Fall of a City, David Gyasi plays Achilles, while Johnny Harris plays Agamemnon. Both men were born in London. Both men are British. Yet the casting of David Gyasi has attracted unceasing criticism, while the casting of Johnny Harris has, to my knowledge, gone unremarked.

What is the difference between these two men?

The Britishness of the cast categorically excludes any argument that the critics' problem is with ethnic authenticity. Instead, what bothers them, and what prompts questions like your own, is quite simply that black actors do not fit the way we perceive a story that is labelled “Ancient Greece”. It doesn’t matter that the story has only a partial and tenuous connection to everything else we know about Ancient Greece. It doesn’t matter that the story doesn’t belong to us any more than it does to David Gyasi. It doesn’t matter that the story doesn’t present itself as a story about a single ethnicity and culture. In our minds, to put it bluntly, “Ancient Greece” is a white space. The intrusion of non-whites into this space makes many commentators bristle. Terms like “historical accuracy” are used as weapons in their fight to return to their perceived version of this mythical-historical environment. This is not actually a question of historical evidence. It is a question of ingrained perceptions that impoverish our creativity and limit our understanding of modern retellings of old stories.