The SR71 was painted in black, but just no ordinary black!
The paint contains, amongst other STILL TOP SECRET ingredients, carbon. All the ingredients together actually work at radar return minimisation, ergo stealth.
The F117 Nighthawk is painted the same for the exact same reasons.
Problem I believe the paint is VERY matt, and as such is NOT a good aerodynamic paint as it is not gloss. That is not a factor in the SR71 because it darned well can outrun and outfly just about anything anyway, and the F117 is slow anyway, and does not need speed as it's defence, instead flying at night (usually), where visual non-radar tracking of it is next to impossible.
There was a NASA SR71 painted gloss white at one stage, and I believe it had a reasonable speed margin over the military examples.