I've read in the Make magazine that LEDs can sense darkness. How should I use it in a breadboard circuit? Is it possible to read the ambient light level and send it to the computer? How does this work? Does that mean a speaker can be a microphone? In that case, how would you do that?
A led acts a light detecting diode as well i.e you can sense the extent of darkness and brightness by a LED but it is not going to be as effective as the one's made for this purpose specifically.
You can make it on your own and this instructable will prove to be helpful.
And answering your second question, yes the speaker can also act as a microphone based on this concept only.
Taking this question literally, I don't think you can sense complete visual darkness with an LED. A human eye can respond to less than 100 photons, so I think you'd need something like a photomultiplier tube, silicon photomultiplier or (maybe) a cooled avalanche photodiode to detect the presence of total darkness.
Sensing normal room ambient light levels is very easily done with a dedicated chip such as those from Taos (née Texas Instruments) which give you a digital output and has built-in filtering to knock out mains frequency-related brightness variations.