Well then, without some basic JASS knowledge it would be really hard to do what I recommended, I think. Not sure if GUI can examine Z levels actually.
Basically you would want to make a trigger for your blink spell, and whenever it is cast, create a loop that checks every say 25 units of distance between the caster and the target point, if any of these points have a Z level that is too much higher/lower than the caster's Z level, then the spell gets cancelled (move the unit to his current position).