space time - What shape are black holes theorized to be?

It's complicated, and it depends on what you mean by "black hole".

In general, for a distant observer, the "black hole" is the event horizon - the surface within which there is no escape to the outside. The more massive the black hole, the bigger the event horizon.

http://en.wikipedia.org/wiki/Event_horizon#Event_horizon_of_a_black_hole

If the black hole is not rotating, the event horizon is a sphere, plain and simple. In a perfect vacuum, and if the black hole is big enough (is not microscopic), looking at it you'll see a perfectly black, round "thing", with the stellar background behind it.

However, the background image near the black hole would be distorted as if seen through a bad lens. This would become apparent as you move around it. This is because the powerful gravity of the BH bends the rays of light. See the first half of this video:

https://www.youtube.com/watch?v=8s-pWPqFQBE

If the black hole is not in a perfect vacuum, but is surrounded by various pieces of matter, it may capture that matter, and an accretion disk will grow around it.

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If the black hole is rotating, things are a bit more complicated. This is an object described by the Kerr metric, and it has two important surfaces: the usual event horizon, and the outer skin of the ergosphere:

Nothing can escape from within the event horizon. It is possible to escape from within the ergosphere if you rotate with the black hole - this is known as the Penrose process.

I was not able to find visualisations of a Kerr (rotating) black hole, but it should look somewhat similar to a non-rotating one, except it would appear "flattened" at the poles, and bulging at the equator.