The best I have found is Ace Hardware's flat black lacquer.
Goes on smooth, cleans well, doesn't show smudges when handled, and once again can be wiped clean with a damp rag.
Black is a difficult color to maintain with most "paints".
Fast cure (again, lacquer) and cures hard so drivers and gaskets don't stick if you have to go back in.
I've pulled drivers from cabs I've had buttoned up for several years and the seals break clean.
No other paint will do that, I guaranty it, besides lacquer, unless you bake it or let it cure hard for days, weeks or months.
And even then............?
Covers very well too.
I use it exclusively on cabs where India ink touch ups won't do.
Also, since it's lacquer, when dry the original texture(s) of what's been sprayed remain.
In other words, it's hard to tell you've even done any paint work if you do a good job of masking.
Except for the fact that you will now have a perfect, blem free finish on 35-40 years old speakers.
Thomas