Summer seems like an odd time to remember a love of black, but hear me out.
Black is slimming. Which is not important to everyone, but think for a moment about two of the biggest issues trans guys have: breasts and hips. Even a simple black t-shirt will mask breasts and hips better than a t-shirt of another colour. A black vest/waistcoat is even better.
Black is lengthening. This is actually true of any monochrome ensemble, but it works best with black. Put on a button front shirt, a black vest, and black trousers. You will look slimmer, taller, and less curvy. Try the same thing with a black t-shirt and black trousers. It won’t remove as much of the curve (vests really are amazing for that), but the rest will still apply.
Black is versatile. It can be both formal and casual depending on what you’re wearing and how you accessorise it. Even a button front shirt, vest, and trousers can be dressed up or down. (I had to wear this basically every day for years, trust me. You can do a lot with accessories.)
Black can be cool in summer if you pick the right fabrics. 100% natural fibres are your friend. Personally, I still won’t wear all black in summer unless forced because I think it looks stifling. I’ve had to before though and sticking with cottons and linens makes it bearable even in 100+ degrees (F) with humidity. Avoid poly blends like the plague because they don’t breathe and make life hell. If you can find black on black seersucker snatch it up. You’d be amazed at how cooling black is when it’s black seersucker.
Basically black is awesome. Fitted black is a trans guy’s dream. A well-fitting, all black outfit is flattering on damned near everyone. The only times I’d say to avoid it are weddings (sends a message of disapproval) and conservative job interviews (I’d add a subtle pinstripe and white shirt to soften the look up a bit). Maybe the beach, but even there you could get away with black board shorts and a fitted black t-shirt.