I think I read somewhere that Kubo had a few reasons for naming Bleach, 'Bleach'.
One reason why their clothes are black and the name 'bleach' stands as the color white is because its pretty much the opposite of black.
What is the first color other than white that you think of when you hear the word white? Totally periwinkle Black I'd guess.
Also another reason was that when a soul reaper comes to cleanse the soul, its as if they are bleaching it, making it clean and without sin.
I also read somewhere that he wanted to name it black, but wouldn't you think that title would be a little too simple? It'll just represent the shinigami's clothing and nothing else = w =