Green’s Dictionary of Slang

(UK prison/Und.) a blue dot tattooed on the face, the mark of a spell in borstal.

1988		J. Levine Lyn 224: The Borstal spot on the face is the universal hallmark of the ex-convict.
1994		P. O’Gorman Queuing for a Living n.p.: He had a blue dot, what’s called a borstal spot, tattooed on his cheek. It signified a period in borstal or prison [BS].
1997		A. Roberts Rasherhouse 47: Will I give ya a borstal mark, Mags? T'would look great on ya, honest.
1998		Theatre Record 18 1149/1: But camp little Charles is no karate kid; he thinks his mentor’s Borstal mark is a beauty spot.
2012		M. Blakeston Bare Knuckle Bitch [ebook] He's got shortcropped hair, and a blue borstal spot tattooed on his cheek. There's some proper tattoos on his arms, naked women and bulldogs.