A blood type, also known as blood group, was a classification of blood.
On Earth, A-positive was the most common type, with one third of humanity in 2006 being A-positive. This was exploited by the Sycorax, who used a sample of A-positive blood in Guinevere One to hypnotise all A-positives via blood control. (TV: The Christmas Invasion)
The First Doctor had TL-positive blood. (PROSE: A Big Hand for the Doctor) According to Dr Lomax, the Third Doctor's blood had a blood type unidentifiable as human blood, as shown by its platelet stickiness. (TV: Spearhead from Space)