Assuming that your universe allows for people of colour, having no black characters is far worse. If you decide to only write white characters out of fear of misrepresenting blacks, then by the same logic you would have to write only men out of fear of misrepresenting women, only (and I am assuming here) hetrosexual, only... &c. &c.
Basically, you would have a universe in which only people who are just like you would exist. This is necessarily racist (sexist, &c.), in that it requires yourself and your readers to imagine only the existence of white folk.
On the other hand, writing outside your experience acknowledges that other people exist. Now this can be rasist if you write token characters, which allow yourself and readers to imagine only the barest of stereotypes, which is hardly better than non-existence. But it doesn't have to be racist, because you can write good characters, and you and your readers will see them as actual people.
I don't buy the "as long as the characters are good it doesn't matter" argument, either. If ignoring the race of your characters leads you to write a white universe, this speaks more to an ingrained racial bias than it does neutrality. The fact that you have picked up on your omission and have decided that there is a potential problem means that you can do something about it, and I applaud you for that.
