I have the same issue, and my Ri second toe was one of the reasons I abandoned my thru-hike in 2011. It had devolved from black, to the skin breaking down and the end of my toe looking like raw meat (which it was…)
Before starting again this year, I went to a podiatrist. He said he thought it was dyshidrotic eczema. But he referred me to a dermatologist, who thought it was an auto-immune disease and took a tissue sample for biopsy. Today I am seeing a vascular surgeon. I still have no idea what causes it (correct length shoe, and 4E width - bunions) but it does resemble a blister (tho not from friction as you would see on the heel or top of the toes).
I suggest you try a gel cot (sort of a sleeve that slips over your toe) to see if that helps any; and be sure that even going downhill, your toes do not contact the front of the shoe.
TicTac