The only people advocating that you paint brick, let alone brick in a fireplace, are painters and the Retail Paint Stores. (It's their business, and they'll do it w/o even telling you why their own fireplaces or home's brick isn't painted.)
Brick is meant to breath. You're turning a maintenance-free material into one that now requires maintenance. The next thing you know, we'll be seeing a Post from you that asks, "How do I get the ugly soot off of my painted brick?" You will DEFINITELY NOT get any sympathy at that time.
I know it's not what you want to hear but either live with the brick as is, change the color of the walls and/or rugs and/or furniture, or have the fireplace redone. If you want to see what folks have to say about painted brick, use your Browser's FIND (or Search) feature and key in the word brick (on the main Forum Page.) Don't stop looking until you either find someone who wants to do what you want to do, or someone who disagrees w/me. My best to ya and hope this helps.