The garment in question you’re referring to actually has a specific name: a royal mantle (or just a mantle. If worn specifically at coronations, it would be called a coronation mantle. Fairly self explanatory). The black-speckled fur you’re referring to also has a name: ermine.

There are several reasons that this specific fur and colour were chosen for royal mantles, and to find out why we’re going to have to look at the Black Death. As a result of around a third of the European population dying in a matter of years, peasants found demand for work was much higher than it had ever been before. Using this as an opportunity, many labourers abandoned the feudal lands their families had been tied to for generations and demanded more generous pay from their overlords.

This was extremely shocking to the feudal lords and nobility of Medieval Europe. According to European Christian tradition, God allocated each person a position in life and to demand better than this was tantamount to blasphemy in the eyes of many, as it suggested God’s placement of you in society was wrong.

Affronted, the nobility struck back at the ‘over-mighty’ peasantry by enforcing a strict set of restrictions on everything from what they could eat to how they could dress - these restrictions were called Sumptuary Laws. They had existed since the times of Roman conquest but were significantly amplified throughout the Medieval period (England, for example, had a comprehensive set of laws in place by 1377).

These laws didn’t, however, just apply to the peasantry. Restrictions were set on every order of society, access to luxury increasing with rank. Now we come to the topic of the royal mantle. Purple was a colour reserved exclusively for royalty, and as such to have a purple mantle was indicative of the fact that you belonged to such a rank yourself.

Similarly, ermine was not only reserved for higher orders of the nobility but also extremely rare and expensive. Therefore, to have long robes in luxurious and expensive materials such as velvet (another textile reserved for royalty in many European countries) trimmed in ermine was to flaunt power, wealth and extravagance.

Hope this was helpful in answering your question!