Tarowyn said:
SA with physical blue magic still make them crit.
As for naturally, it's like WS's in that it has to have crit in the description for it to be able to crit. Sickle slash is the only one I can think of off the top of my head that has it.
May not be true for all weapon skills, but I know that even with out "chance to crit varies by tp" weaponskills can still critical (example, penta thrust, which has "accuracy varies with tp")
As for blue magic, my guess would be yes, since for example, black magic can critical hit, and witch sash slightly improves that.