A DARK KNIGHT...

First, my apologies if I come off as an art teacher. Second, please don't take my critique as a criticism - that's one problem I always seem to run into.

I think Batman looks pretty sweet in general - I like the retro blue batsuit and I think the gold works nicely with it. My only problem overall is that the title of this piece is "A Dark Knight" and I don't feel that that is effectively conveyed in the finished work (even though it's "Knight" not "Night").
Batman is, more often than not, portrayed as dark and brooding; he is vengeance, on the prowl for scum and villainy; he seeks retribution for his parent's death and justice - often violently - and he is, after all, "the night" (BtAS, 1995).

While I do like how you've successfully created the rain effect and I enjoy the watercolour technique you've been working with recently, I think that it looks more lile Bats is out on a rainy afternoon than prowling on a rooftop at night. In future, I might suggest going from dark to light from top to bottom, rather than from bottom to top - this might help to create the illusion of a night sky, with the lights of a city coming from below. Otherwise, looks good, sir!