Just watch the Blood percentage at the top of the screen. It will tell you something like 0/60, with 60 being whatever percent of all blood on screen that you have to collect to accomplish the mission. Just keep cleaning past that number until you get to 100/60.
Some general advice: First, keep in mind that several hiding places give you an option of where you exit. The truck cab outside the gas station, for example, is much more useful because you can get in on one side and out on the other. Just push the stick to the desired side before exiting. Second, remember that some of the blood pools are dynamically placed. If you just can't get one, restart the mission and see if it moves to a more advantageous spot.