just my two coppers...
If you're not using the psionics handbook, then Mind Blast is a spell like ability and potentially subject to Spell Immunity the spell.
If you are using the psionic handbook (or some psi system), then either:
The default "magic is the same as psionics" assumption is made, and spell immunity work may work.
or:
The "magic and psionics are different" option is used, and spell immunity does not apply.
If Spell Immunity (the spell) potentially applies, then I'd suggest deriving the level from the power point cost of the psionic handbook version of mind blast. It costs 9 power points to use (as a psionic attack) which equates to a 5th level spell. Not subject to Spell Immunity the spell, but it is subject to Spell Immunity the special quality.
(Edited again - you meant the spell, not the special quality, didn't you? I really shouldn't post when I'm tired.)