I just finished painting a light-tight enclosure and used the Krylon 4290 Camouflage ultra-flat black paint. I've found it to work well in the past.
I also have a bottle of a special flat black acryllic paint that Gary Walker gave me: Blk3.0 from culturehustle.com. A similar product (Black 2.0) is available from the Stuart Semple brand through Amazon and other online stores. I've also used the Testors EPM25-2549 flat black marking pen when doing things like blackening filter edges.