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.
Thank God For Kids