I am not sure if my toenails turned black because they always have polish on them (I started this a year ago) and a few months ago, one fell off...then the other. I thought I might of gotten a foot fungus but my second toes are longer than my first and after one ride with thicker socks, I noticed the top of the toes kind of hurt. Now I make sure I only ride with the 'sock guy' type socks not the thick running socks...