One of the more authorative sources, 'Loyalty Is My Honour - personal accounts from the Waffen-SS'*, makes no mention of a tattoo blood group.
That said, if it is a myth, it goes back to the seventies at least, as it featured in at least one comic story
*brilliant book, and one thing that does stick out - training really was harder the further back you go! Although the entrance standards and training declined as the war wore on, the initial selection tests included a 2mile run in 'full kit' in under 20mins. Sounds easy? Full kit weighed 220lbs...!