In normal circumstances the ruling is that if blood mixes with saliva from within the mouth, then if the saliva is more than the blood (a sign of which is that the colour is whitish or yellowish) than the fast will not break if the saliva is swallowed. However, if the blood is a greater amount (where the saliva is a reddish colour) then the fast will break if the saliva is swallowed. Furthermore, if both are in equal proportions, then too the fast will break.1
Similarly, if the taste of blood is discernable when swallowing than too the fast will break, irrespective of the colour.