[igotrhythm]

Originally Posted by TC_Halogen

You'd be looking way back into rhythm gaming history here to know why that term was coined. It seems to carry over from a lot of rhythm games, but many people leave out the color and tend to generalize a single non-perfect judgment as a "flag."

It's something you might be able to look up, though.

Pretty much this. The way I heard it, the term was originally applied to having a perfect score tarnished at the last possible moment, but later expanded to mean any run where one unfortunate judgment holds a player back from the highest possible score on the song. Later, the "flag" terminology spread from DDR (?) to other rhythm games, since there's a fair amount of overlap in the larger community.

To be more specific to how it applies here:
Blackflag is 1-0-0-0.
Avflag is 0-1-0-0.
Missflag is 0-0-1-0.
Booflag is 0-0-0-1.