I have used black switches for almost a year (stopped use them just recently), and I had no real problem with them since I mostly game and just write smaller bits of texts (kinda like this forum post).
If I had written some kind of big and long text I think I would have fatigued a little bit.
Even though I didnt fatigue really with blacks while gaming etc, now that I use reds I feel....... relief? when I just generally use them, both for typing and gaming, it feels so much better and smoother.
And when Im going back to blacks I feel in my fingers that they slightly ache.