Hate it when that happens
Half black face, and I assume you mean one side of the head, is terminal IME. Someone told me it's caused by nerve damage - have seen it twice in 4 years, fish lasted about 1-2 weeks in each case and then went downhill fast and died within 24 hours. Could also indicate a circulatory problem I suppose, hence the discoloration (remember bilateral symmetry), unlikely there's any treatment in either case, good chance it's a result of physical trauma (like smashing into something).