Im not really sure, but Ive had a 0 blacksmith reach level 5 skill, after a while I look and there is 0. I think I loaded the game and there it was.If it still gave progression to his overall level, then it would be easily exploited. U know, reach skill 1 in BS, it gives a point, load, so BS is again 0 and give it another go. I didnt try it however.
Heroes are already super - skilled and progression is nonexistent at higher levels due to gimped maths that give exp, like giving 10x instead of 220x as is written in detail.