Guys thanks for the comments,
Think you guys are correct - I've washed the damn boat down so much there couldn't possibly be any trace of blush left. I rolled the epoxy coats on at the same time so no temp differentials between sides. It all feels the same to the touch so think there must have been a pocket of uncured stuff in there somewhere. Sandpaper in the ROS just clogs up with black gunge. Don't know way forward now - I've scubbed the living daylights out of it so am out of ideas. Luckily it's on the hull i suppose - maybe I'll flip it over try and get the deck in shape and revisit it next week?...