In all honesty I have used the term the black fella to ask about someone whose name I don't know and also the white guy.
For example we have 4 regular delivery drivers from a supplier 3 white, 1 black, I only know one drivers name so when asking or commenting on the last delivery I will say the tall black fella/lad or the small dark haired white guy I do use both black and white equally as a descriptive form for an unknown person, sometimes with another descriprion (tall, fat, with glasses etc.)
Seems Ba objection was that he used the term black guy, in a group of 3-4 black men, but as said not all facts are yet established.
Could easily be a misunderstanding or the 4th offiicial had said more, hence the refusal to return to the pitch, if a slight misunderstanding surely it could have been sorted out and the game resume.
In the end if Uefa keep to form they will sweep it under the carpet as always