Many of the older generations in my family were granite masons and quarrymen and the word "segs" in my experience meant the blue-coloured scars resulting from the inclusions of dirt (in their case usually stone-dust) in the cut which remained after healing.
When I left home in Cornwall and went to Yorkshire for a few years I was surprised to find my landlord using the same word with the same meaning, though in his case, as overman in a colliery near Barnsley, he meant the blue marks and scars retained by coal-miners from cuts suffered underground. I had thought, up to then, that it was a Cornish-dialect word but obviously not, there it was alive and well and living in Yorkshire!
I had never encountered the term as meaning hard skin, a callus, or scars with a hard callus surrounding them, though that seems to be the general concensus here in this thread. Interesting!
Oh yes and I had Blakey's segs on my shoes too, but that's a different story altogether!