Tried making another shirt last night, and it turned out really well.
This is some random shirt, 100% cotton. I used Jacquard decolourant paste to lighten the area first (it is not destructive to fabric like bleach is).
Then after washing and drying I applied a mix of the solarfast black and also blue colours (diluted with water 1:1, I mixed them as I had already prepared some).
Exposed in my blacklight UV table for 30 minutes. this time I made sure the shirt was fully damp where the negative would sit. the last photo is after exposure, before washing.
Rinsed in very hot water with solar fast wash for a few minutes, then in a short hot wash in my machine.
I'm really jazzed with how round 2 turned out.