Let's say that Person A is on a planet orbiting a black hole (like the one from Interstellar) and time is slowed for Person A. Person B is back home on Earth. Now let's say a supernova happens and it's possible for both parties to see it happen at the same time (ignoring the time it would take for the light to get there). Would Person A see the supernova at the same time as Person B? And if so, if Person A is in 1,000 B.C. and Person B is in 2,000 A.D, would the supernova then happen at two different times at the same time?
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