it's E flat, and I threw in a major 7th (D#) for a good diminshed 7th feel to keep things interesting(I built a D#, F#, A chord). It's too weird to be a diminished 5th (that's just a normal tritone found in any blues scale).
Love the synth you introduce around
1:20
. The melody at the beginning feels weird because you're using a color note that clashes with the accompaniment. It works with the bass because it has enough attitude that throwing in that color tone (I'm not positive but I think it's a diminished 5th) works.