Why are light bulbs black body radiation emitters? If the definition of a black body is:
"A blackbody is an object that absorbs all of the radiation that it receives (that is, it does not reflect any light, nor does it allow any light to pass through it and out the other side). The energy that the blackbody absorbs heats it up, and then it will emit its own radiation."
Then why are light bulbs black bodies? Don't they generate heat themselves instead of absorbing radiation, then giving radiation out?
 A: "The energy that the blackbody absorbs heats it up, and then it will emit its own radiation." --- This part may be a bit misleading.  It should read as

The energy that the blackbody abosrbs heats it up, and will be emitted as part of its own radiation.

In fact as long as all radiation is absorbed, i.e. not reflected/passed through, it is a black body.  A black body does not need to absorb heat first, then to begin radiating out.  A light bulb can be roughly viewed as a black body; as a result if you subject it to radiations with a temperature far higher than its own typical temperature, it would heat up to (possibly more than) that one.
A: I think your perplexity arises since the definition your cited is easy to misunderstand if you don't look at it carefully.
Let's try and break it in two parts:


*

*A black body is a body which absorbs all the radiation it receives (or, equivalently, reflects none of it).

*A black body is a body that gets heated by the energy it absorbs, and emits thermal radiation accordingly (indeed, black body radiation if it's in thermal equilibrium with the environment).
If you look carefully, it doesn't say that such a body is heated just by the radiation it absorbs. It says energy.
In the specific case, a powered-on light bulb get just a very tiny fraction of the energy that heats it up in the form of radiation from the external evironment. All the rest (almost all) is supplied by the means of an electric current carried by a wire: the electrons strike the tungsten atoms making them to vibrate furiously, and you know heat is just kinetic energy.
This explains why the filament (NOT the whole bulb) is a good approximation of a black body. It's not perfect though, as other users mentioned, since it's not a perfect absorber (perfect absorbers do not exist in nature, as far as we know).
Note that the light bulb filament is no longer a good approximation of a black body once you turn it off.
Another (even better) good approximation of a black body is the surface of the Sun (or of a star whatsoever). It does not certainly emits so much EM energy because it's heated by the radiation absorbed from other sources (that is present nonetheless). Rather it is heated by the fusion occurring hundreds of thousands kilometers below.
