You could maybe paint on some black on the opposite diagonal corner - lower right on the picture - to sort of balance it out. I'd be worried about getting black where I didn't want it, but I've had some success with touch-ups like that, where I mix some dye and soda ash to make a strong dye solution, dampen the garment with a spray bottle (I usually dye damp, and if I don't touch up damp the dye creeps differently), and go for it, trying to make the new part fit into the design, like I'd meant to do it that way to start with.
You could cut off the sleeves, too.

I'm not above that sort of thing, especially in the hot summer we're having this year!
Thanks for your comments, deb.
I hadn't thought about "blackening" the opposite corner, but that's a pretty good idea.
The real problem with the black on this T-shirt, I've decided, is that I didn't use enough of it. I wanted to dye
everything outside the two spirals black, but didn't use enough. You can sort of see what the result would have been, as there is the beginning of a lazy dark curve running between the spirals. I think the shirt would have looked better if it had been more distinct/darker/blacker.
Still, I wore the T-shirt for the first time at a BBQ at my sister's yesterday, and the one compliment she gave it was: "I really like the black sleeve."
You never know!
Tie-Dye Todd