All members of the Waffen-SS were required to have a tattoo on his left arm verifying his blood group. Note that not all members actually had a tattoo even though it was required, this included any of the high ranking officers and those who joined the Waffen-SS in the later part of the war. The tattoo was normally applied to those who did their basic training in the Waffen-SS, but also non-Waffen-SS soldiers could get the tattoo if they were treated in the Waffen-SS field hospitals.

The purpose of the tattoo was to be albe to perform a blood transfusion at the front to save a wounded mans life. The blood group was also mentioned in the personnel-files and his ID-papers.
Two different types of tattoos existed, one in gothic lettering and one in latin lettering, the latter one being used later in the war.
The tatto was about 7 mm in length and was placed on the underside of the left arm, about 20 cm up from the elbow.

The tattoos were used after the war to identify SS-members and many former members (and Waffen-SS soldiers) removed the tattoo to be able to hide their SS-past for the allies.

A soldier from 13. Waffen-Gebirgs-Division der SS Handschar spoke of how he removed his tattoo:

Later, in the prisoner-of-war camp at Tamsweg, I met Sturmbannführer Liecke from our division. He brought us to a physician from the 14th SS Division, who provided us with hydrogen tablets to remove our blood type tattoos. We moistened these tablets and dabbed them on our arms. This was extremely irritating to the skin, but the tattoos simply came off after two to three days. Naturally the skin required about two to three weeks to heal. Because of these tablets, I was successful in passing two inspections conducted by our captors. (1)

Footnotes
1. "Himmler's Bosnian Division: The Waffen-SS Handschar Division 1943-1945" by George Lepre, page 310.