How does one get a blister under the toenail? Or rather, how does one avoid such a thing?
You can lose a toe nail if your footwear is small or your nails are too long. The toe nails hit against the front of your footwear thus they turn black and can fall off. Btw it´s very painful.
Other than dropping something onto your toe, @
MendiWalker is right. Long toenails and shoes that are too tight or too short would seem to be the major risk factors when on the camino.
How to avoid this? Before you go, get your footwear fitted properly. If in doubt about a shorter or longer boot/shoe - defer to the longer size - you can add padding to your foot within reason, but it really isn't possible to shorten them.
Once walking, make sure your toenails are trimmed regularly.
On the trail itself, there is at least one simple technique to reduce the risk of your toenails hitting the front of your footwear, and that is to stop at the top of steeper descents, and re-tighten your boots/shoes as tightly as you can. At the bottom, stop and re-adjust the lacing - you don't want to be walking in tight boots/shoes all the time.
If you are still finding problems, find thinner socks if you can. This may be enough to save having to go all the way and buying new boots/shoes that fit properly.
Regards,