Blood control was a method of controlling beings via their blood type.
On Christmas morning 2006, the Sycorax used blood control against one third of the population of Earth. They captured the Earth probe Guinevere One, which carried a sample of A-positive human blood. They inserted the sample into a control matrix, which sent out signals that had to be obeyed by all those with that blood type. In this way, they were able to make all the people on Earth with A-positive blood stand on top of roofs, apparently ready to jump to their deaths.
On this occasion, the Tenth Doctor called blood control a "cheap bit of voodoo", and likened it to hypnosis. Blood control, he said, could not overcome the basic human survival instinct. Although Earth's A-positive population might have appeared to have been in danger of jumping off roofs, in truth blood control was not strong enough to have actually made them jump. (TV: The Christmas Invasion)
However, despite this flaw, the Seventh Doctor said that if they had a blood sample from every member of the race they intended to conquer, they would actually be able to exert absolute control over the entire race. (AUDIO: Harvest of the Sycorax)