Of course, I know that, I used a blacksmith before
So scenario:
1. I take a lvl75 set item (chest) with a gold slot
2. Open my blacksmithing (lvl1) and try to put a rune in the slot
3. All slots go red and I can't put it in (can't put anything in there, tried rune, ring, amulet and chunk of lava).
4. Visit a blacksmith and put the same rune in the same slot no problem.
(the same thing happens for any item not just the chest in question. I didn't try low level items in it tho)