Marius Black was the great uncle of Sirius Black and a Squib. Marius, the son of Cygnus and Violetta Black, was born between 1915 and 1920 (probably 1917 or 1918). He was the brother of Pollux Black, the father of Walburga Black, Sirius and Regulus’s mother. Further details of his life are unknown because he was blasted off the Black family tree tapestry by Walburga (BFT).
We don't know the actual year that Marius was born since his name and associated details were burned off the tapestry (and appear on Rowling's version as a gray blob. However, we can determine that he was born between his sister Cassiopeia in 1915 and his sister Dorea in 1920.
We can use some logical guesswork to narrow those dates down a little more. Cygnus and Violetta has children about every 2-3 years, it would seem. Since Cassiopeia was born in 1915, we can add a couple years to place Marius's birth in 1917 or 1918. Then a couple years after that, Dorea was born in 1920. We'll place Marius's birth as circa 1917.
Family
Marius's father Cygnus was born in 1889. His mother was Violetta Bulstrode. His younger sister Dorea married Charlus Potter while his older brother, Pollux , married Irma Crabbe. Cassiopia Black, his older sister, was unmarried. Marius was the great uncle of Sirius Black.
Commentary
Etymology
The name Marius has no connection to stars or constellations, which is unusual for the Black family.