I managed to find a workaround for my project’s black screen on start → I added a blank level to be loaded first and then it loads my main level. I believe if I had also delayed the spawning of my pawn it would also work - it appears to be some sort of race condition as a guess causing the problem.
Hopefully this helps anyone else who stumbles on this in 4.15 - if you see your loading screen or your begin play debug text stuck on the screen flashing then try adding a delay either by a blank level or delayed pawn spawn.