This question is based on something of an incorrect premise.

First of all, the asker of this question is clearly using the term "sub-Saharan Africans," which nominally denotes people who live in the geographic region of the African continent south of the Sahara, as a euphemism for "Black people." The usage of this term in this manner, however, bears the demonstrably factually inaccurate implication that, in ancient times, people with features that twenty-first-century westerners would typically consider Black only existed south of the Sahara Desert and that no people in North Africa in ancient times ever possessed these features.

White people with racist prejudices and agendas frequently deliberately use this term in this manner in order to try to inaccurately portray the ancient Egyptians, Carthaginians, and other ancient North African civilizations that are popularly considered to have been highly "advanced" as homogeneously white (or, at the very least, homogeneously non-Black) and the Sahara as an impenetrable barrier preventing any people originating south of it from ever going north of it.

The reality is far more complicated than this white supremacist fantasy portrays. No part of North Africa has ever been racially homogeneous. People with very dark skin and features that most twenty-first-century westerners would generally consider Black have always made up a significant portion of the population in North Africa and, indeed, have long had at least some presence in southern Europe as well.

Leaving that aside, the ancient Greeks were reasonably well aware of the existence of lands inhabited by mostly dark-skinned peoples from the very earliest recorded history. The so-called "Captain of the Blacks" fresco (which is very misleadingly and problematically named) from the Hall of the Frescoes at the site of Knossos on the island of Krete, dating to between c. 1350 and c. 1300 BCE, already depicts a man with black skin and afro-textured hair, clearly demonstrating that people in the Aegean world were already aware of the existence of Black people as early as the fourteenth century BCE and further suggesting that some Black people may have already been present in the Aegean at this early date.

Black people are mentioned in Greek literature from almost the very beginning. The Odyssey is an ancient Greek epic poem in dactylic hexameter verse which probably became fixed in something resembling the form in which it has been preserved to the present day sometime around the second quarter of the seventh century BCE. It is one of the earliest surviving works of ancient Greek literature. The following passage occurs near the very beginning of the poem (at Od. 1.22–26):

"ἀλλ᾽ ὁ μὲν Αἰθίοπας μετεκίαθε τηλόθ᾽ ἐόντας,
Αἰθίοπας τοὶ διχθὰ δεδαίαται, ἔσχατοι ἀνδρῶν,
οἱ μὲν δυσομένου Ὑπερίονος οἱ δ᾽ ἀνιόντος,
ἀντιόων ταύρων τε καὶ ἀρνειῶν ἑκατόμβης.
ἔνθ᾽ ὅ γ᾽ ἐτέρπετο δαιτὶ παρήμενος:"

This means, in my own translation:

"But he [i.e., Poseidon] had gone among the Aithiopians, who are far away,
the Aithiopians who are divided into two groups, the most distant of men—
they are where the son of Hyperion [i.e., Helios, the sun] sets and where he rises—
going to meet with a hekatomb of bulls and rams.
There he at least was joyful, sitting alongside the banquet."

The ancient Greeks applied the name "Aithiopians" to dark-skinned people whom they regarded as inhabiting or originating from a very far-off land. The poet of the Odyssey describes them as living in the far west and far east, but later Greek authors identify them with the peoples of the lands south of Egypt in what is now Sudan.

For more information about this topic, I highly recommend the work of Sarah F. Derbew, an assistant professor of classics at Stanford University, who has written extensively about Black people and representations of Blackness in the ancient Greek world. She published an excellent and very accessible essay in Aeon magazine in March 2022 about ancient Greek artistic depictions of Black people and she just published an excellent monograph on the subject of ancient Greek representations of Black people in general in April 2022 through Cambridge University Press titled Untangling Blackness in Greek Antiquity.