Blackjack seems to have been a one-off DIY label. Presumably it was run by The Blackjacks, who were responsible for what appears to have been the only record on the label, a single called 'Thank God For Kids' (BLJ-1; 1974).  The 'B' side was in Welsh - 'Diolch Am Blant' - which suggests that the band and their label may have been from that country. Thanks to Robert Lyons For The Info.

74 The Blackjacks Thank God For Kids  BLACK JACK  BLJ 1