Not quite specifically his suit, but the legion uniform I reckon
Yeah that's what I was thinking, though I would still probably go for another color for the Legion uniforms anyway. The DEO wears black, as well as Reign, Psi and to a lesser extent Livewire (she incorporates some blue into her outfit). So I would probably try to find something that would stand out from the DEO black, and pair well with Supergirl's primary colors. (She is their inspiration after all ) so maybe, dark grey or dark red with black piping and navy blue accents?
I guess they want the legion to blend in with the DEO, or maybe they just like black.
Don't assume malice when stupidity is an adequate explanation. At least, not the first time.