I understand black is the hot wire from the panel box, but it seems that if all the hot wires coming out of the panel box are black there can be confusion.
When using EMT it seems that if there are three circuits in each raceway, if each circuit hot were a different color, say black, red, and blue, then it would make identifying the circuits easier.

Today an electrician told me that he uses black out of the panel box for phase A and red for phase B.

What do the pros recommend?

For Residential, Red and Black to identify Phase is good, can be all blacks or all reds.Multi circuits where 2 hots on oppsite phases share a neutral, red and black make it easy.
3 Phase, you may want black red and blue.
If it is romex, it will be mostly blacks.