[potential bug] Blasphemy's hex gem level requirement was not shown anywhere

I just try to swap my Temporal Chain for Despair in Blasphemy's support socket, and it report "missing socketed skill" and will not activate (Image below). I check and I have enough spirit for it, and not enough spirit will be report differently anyway.



Then I though maybe because my Despair gem is too high level for my Blasphemy(even though my old Temporal Chain is already 3 level higher than it), so I get a level 10 skill gem and cut Despair into it to test. And it work:



Is this bug, or is there a hidden level requirement that was not shown? And what is the level restriction? My Blasphemy is level 15, Temporal Chain is 18 level when socketed (16+2), first Despair is 22 (16+6), and second despair is 16(10+6)? So what is Blasphemy support requirement? Equal to its level? Its level +3? How can I found that out?