When you cast Winterthorn Blessing on a target other than Ivy, while only targeting a creature you control and opting to not target any creature you don't control, the trigger from Ivy does happen, but the spell automatically fizzles. The reading of the card says the copy has to target Ivy, it doesn't impose a restriction on the spell not also targeting another target while it targets Ivy, provided that the triggering spell had only a single target.

This second part might be wrong based on some obscure ruling, but I don't see why Winterthorn Blessing couldn't follow the 1 target restriction as you can clearly cast the spell with a single target.

(This interaction also triggers the upside down card glitch that just stays there for the rest of the game as if the stack was never fully cleared)