Also, Hex gives disadvantage on ability checks, not saves. So even if you cast something that required a Dex Save (instead of a ranged attack as @ash elemental mentioned), hex would have no effect on your chances to hit.
To my extent of (limited) knowledge the chance to hit shouldn't change. Eldritch blast is a ranged attack, so it is defended by the imp's armor class, not ability checks.
Great, thanks for the info.
