I can't say what was the ethnic composition of the Serbian army at Chernomen, though I also believe that if there were common soldiers (levies), they would've been mostly Bulgarians and maybe Vlachs, while the professionals and nobles would've been mixed, with probably a Serbian domination (especially in the nobility, which does include the proniars, just as knights in the West are a part of the nobility as well). But that doesn't really matter that much - the difference between Serbians and Bulgarians in those times was minimal - just look at all that enormous amount of Bulgarian-Serbian relations (heck, even how a big part of the Bulgarian royal family went with Elena and so easily became incorporated in the Serbian state). Actually, the 14th century Balkans, while they seem rather divided, also seem rather cosmopolitan - look at the spiritual people (clerics, teachers etc) and how they freely move all around the place, from Bulgaria to Byzantium, to Serbia, to Wallachia and Moldova, serving everywhere despite their origins.
Anyway, what I find interesting about Chernomen is some new (or maybe old, but recently repopularized?) analyses, which suggest that Ioan Shishman might have been in coalition with Vukashin and Uglesha, since after the battle the Turks (was it Murad ruling over them then?) went against Bulgaria first and soon met a Bulgarian army which they routed (supposedly, late-arriving reinforcements). So, who knows, maybe the Balkan rulers did sporadically try to unite, but they were just damm too drunk to do anything right (remember the description of Sofia's garrison, as well as the recent finding about the "rakinya" drinking?).
P.S. Why would the Mrnjavcevici be Serbian? Weren't they supposed to be of Bosnian origin and only served in Serbia?