If you're near a black hole and your time is slowed down, would a supernova be observable to both you and someone outside of the blackhole's pull?
Time is relative therefore this is a bit of a nonsense question.

But yes, if the gravitational effect near the black hole is strong enough (depends on how close you are) it would affect 'when' the observer would notice it.  This is because the light must travel a 'further' distance through the spacetime to get to you since gravity is distorting it.

You may find this interesting: https://web.archive.org/web/20130424041627/http://www.thebigview.com/spacetime/spacetime.html