Which son are you talking about? I didn't know blackwing had a particular son. Unless you are talking about Deathwing having a son (Nefarian, aka blackwing). I'm going to assume that you meant that.
Anyway,
Deathwing had sex with
Sinestra, thus impregnating her. It occurred in shadowmoon valley, in present day "The Hand of Gul'dan". (see map)
Naturally, dragons prefer higher altitudes when they are making babies because there is less air resistance, making it easier to maneuver and move around (same reason why airplanes benefit from flying so high). Deathwing has a couple of signature baby-making moves, so having enough altitude is the greatest importance. It was very hot that day, about 90 degrees fahrenheit, only worsened by the fact that they both breathe fire. Anyway, "The Hand of Gul'dan" was previously just a mountain. The ridges formed from Deathwing and Sinestra going at it. The green lava pools did not exist either. When two dragons attempt to make babies, their fires exchange into each others' bodies, creating this fiery ectoplasmic goo. The goo dripped down from the mountain to the surrounding valleys, filling them up. The catastrophic event lasted 1 day, 2 hours, and 12 minutes.
Three months later, Sinestra took refuge in a cavern somewhere in the valley to lay eggs. Deathwing had already left shadowmoon valley, so Sinestra was all alone to raise her children. She kept them warm with the goo, and then they hatched (Onyxia, Nefarian, Sabellian, and Dargonax). They lived there for a while before parting ways. Nefarian and Onyxia left through the Dark Portal to get to Azeroth. Onyxia found a nice cave in Dustwallow Marsh and made a very convoluted quest chain to enter her domain. Nefarian sought to build an empire, in honor of his father, so he took refuge in Blackrock mountain and formed blackwing lair. He gathered other dragons and made some more babies (he was into the twilight dragons a lot. Nefarian has exotic taste and rebelled against the standard, "Black dragon with black dragon" rule), and hid in his lair.
Hopefully that answers some of your questions. If you need more details, feel free to ask. I have a lot more info, I only wanted to give you the necessary details.