I'm not professional, but I can try help you haha.
I can't test it to you, but you can try it:
Go to src/map/skill.c
Search for this lines:
Change for this:
case 6: // Level 10 Blessing
sc_start(ss, bl, SC_BLESSING, 100, 20, skill_get_time(AL_BLESSING, 20));
So recompile.