I don’t think I’ve ever seen these colors together. I’m thinking of country flags, sports teams, fraternities and sororities, etc. and nothing comes to mind.
Does anyone know anything or any place that has those colors as a theme?
South Korean flag (plus white)
A quick Google indicates there’s a song by The Warriors called “Red, Black, and Blue”.
Sports teams, Greek organizations, etc., often only have two official colors rather than three. There don’t appear to be any regional flags that use only the colors red, black, and blue, possibly because this would be a pretty dark flag without much contrast. There are, however, a few that use red, black, blue, and white, such as South Korea: Category:Black, blue, red, white flags - Wikimedia Commons
Uhh… Bedfordshire, UK Bedfordshire - Wikipedia
As said above, red/black/blue is just a weird combination by itself that doesn’t do really do anything visually.
Huh, East Frisia’s flag is a black, red, blue tri-colour.
orcenio took my answer. 
Also, in addition to East Frisia, Wiki throws up a couple more flags: The Republic of Pontecorvo (which apparently was a Papal State, and it’s a light blue) and Uganda People’s Congress (a political party in Uganda).
(I was going to suggest the polyamory flag, but that has a golden “π” in the center, so I guess it doesn’t count.)
Not much, is it?
The Leather Pride Flag uses these colors, plus one white stripe. I’ve seen versions of it without the white.
The flag of Antigua and Barbuda is mainly red, black, and blue, though it contains white and yellow as well.
Well, there’s this guy.