Well, black tea isn't exactly black. You might say it isn't red either. Nevertheless, I wouldn't get hung up on the color. It's just a name a kind of tea that in English we refer to as black and Japanese refers to as 紅.
Next time you pour yourself a cup of "black" tea in a white porcelain cup, just fill it about a centimeter deep and reflect on the color. It's neither black nor bright red, but it is somewhat ruddy looking. I've always imagined that's where the name of the tea in Japanese came from.
I imagine, but I don't know, that the name of the tea in English comes less from the color of the liquid tea itself as from the color of the leaves: a good English Breakfast or Earl Gray are, well, not exactly black (in fact, rather brown looking to me). But, in English, we tend to see the world in black-and-white.
Here is an interesting link on the naming convention which seems to suggest that "black" is derived from the meaning on Oolong in Chinese. I don't know since, at least in Japanese, Oolong, apart from being written ウーロン茶 can also be written as 烏龍, both of which I've seen. 烏 means crow (so black because crows are black?). At any rate, there seems to be quite a lot you can find with a google search about the different naming conventions used and theorizing about why.