Question: One thing I know about black holes is that an object gets closer to the event horizon, gravitation time dilation make it move more slower from an outside perspective, so that it looks like it take an infinite amount of time for the object to reach the event horizon. It seems like a similar process should slow the formation of the black hole itself: As the star collapses, its gravitational time dilation make itself collapse more slowly. This make me wonder, are what astronomers claim to be black holes really black holes, or are they stars that progressively make themselves more similar to one without actually reaching the stage of having an event horizon?

EDIT: Contemplating one answer, I realize the question is ambiguous. What does finite time mean in general relativity. Here is a less ambiguous question: Is there a connected solution of 3+1 dimensional general relativity with one space-like slice not have a singularity, and another space-like slice having one.

Answer: 
What astronomers claim to be black holes are objects that "progressively make themselves more similar to [a black hole] without actually reaching the stage of having an event horizon", *as they reckon*. That's assuming that GR is valid, since all such claims depend on GR's equations. Plenty of books on GR note that black holes are perhaps better named "frozen stars" from a distant observer's perspective.
It seems like, if there is a sufficient amount of matter localized during the Big Bang, then a Black Hole could form. But after that, a massive star, as seen by the rest of the universe could never shrink to the size of the event horizon.

In other words, a Black Hole would have to preexist in the universe to be an actual Black Hole.