I run into this when I make a pork shoulder or, as here, a fresh ham roast (and when I try to make chiccharones, for that matter). Whether the skin puffs up or simply remains smooth/tough and darkens seems to be entirely hit or miss. In this case, I made a fresh ham. Injected with a seasoned solution (from Kosmo's - great stuff full of glutamates) specifically to keep the brine off the skin. Roasted on low heat (250) for several hours, when the internal temp hit 135, I took it out, turned the heat up to 500, then put it back in till it became clear the skin wasn't going to do anything more than just start to burn.
See the picture - there ARE spots of "fried pork rind" puffy skin, but most of it is not. Anyone know the science behind this and what I can do about it?