OK, I have read a lot about the, supposedly awesome, place that is Blackreach. BUT! How and when do I get there? Is it in the main storyline or...? I wanna check that place out

Start in http://www.uesp.net/wiki/Skyrim:Alftand, SW of Winterhold. It's involved in a pair of related quests.

Blackreach is reached by going to the very bottom of certain large Dwarven/Dwemer ruins, Alftand among them. Once in Blackreach, it is possible to unlock some "lifts", which will allow you to reach it directly from the surface.