I've tried to cast Spells like Blind and Poison multiple times against Boss enemies, yet they always miss. Can these Bosses even be effected by these Spells?
Traditionally Final Fantasy bosses are immune to most negative status effects with specific exceptions (slow being a common-ish one).
Browsing through the final fantasy wiki I don't even see a reference to specific immunities for FF3 bosses (most other games note immunity piecemeal when relevant), so I'd say that's a good hint they're just all immune.
Generally trying to use status effects on bosses is a waste in FF anyway, unless you're aware of a strategy that calls for a status a boss is known to be weak to (or, rarely, if an in-game character suggests their weakness).