I have grown Blackjack a couple of times, bought from Sweet Seeds, and it never had a sniff of purple in it. I have also heard that it was a Jack Herrer Black Domina mix, but it certainly never had the bang that you would get from the Jack Herrer of 90's fame. To be honest, it smells more like Power Plant than anything else. It grows well, doesn't need much care, and clones in about 10 seconds, but the smoke is pretty average. Rumour has it in the Dam that a lot of strains coming out of here are just mixes of Power Plant...which is what most coffeeshops sell as White Widow or JAck Herrer (the rip off coffeeshops that is.)
Anyhoo, great looking weed!!
Peace,
DST