Nice work. Hope you got your airbrush cleaned OK. That's one of the problems with any matt black paint, however good it is - the heavy pigment tends to thicken quickly. I've got a tin of Humbrol matt black enamel that has started to harden in the tin, after only a few weeks, and it's only had about 25% used. Humbrol has got worse for this though, since the new owners changed manufacturer.
One thing that might help for future use with this colour, is to use the Vallejo Model Colour version, which you can thin yourself, which may allow a mix to prevent, or at least slow down the clogging. I'm afraid i can't advise on the possibilities of thinning the ready to use Model Air, as I haven't yet used any. I do like the Vallejo paint I have used though - very nice!