Walaikum assalam,

In the Name of Allah, Most Merciful and Compassionate

The condition for blood [or other impure substances such as pus] to invalidate ones ritual ablutions (wudu) is that they:

a) exit the body; and

b) flow past the point of exit, actually or effective (such that if left unwiped, it would have flowed);

Anything that invalidates ritual ablutions (wudu) is impure. 

That which exits from the body without invalidating wudu is not impure, such as blood that appeared on the body surface but did not flow past the point of exit. 

Ritual ablution (wudu) is nullified by:

1.      Anything that exits from the private parts, even if it not customary such as a worm or stones.

2.      The flowing of any ritually impure substance (f: such as blood or pus) from a wound (f: or other opening) to any place that requires cleaning (f: i.e. it flowed beyond its point of exit), or if it came out with enough force to flow (f: but was prevent from flowing because it was wiped or absorbed by a bandage).

3.      Vomit that is a mouthful, such that one would not have been able to keep it in without difficulty, except if it is just phlegm. Vomit that is from the same motion (ghathayan) is considered together when determining whether it is a mouthful or not.

4.      Blood (B: from the mouth or throat) that overcomes ones saliva or equals it (f: if closer to red, wudu is invalidated; if closer to yellow, it is not).

5.      The sleep of one whose rear is not firmly seated. As for the one whose rear is firmly seated, his ablution is not nullified by sleep, even if leaning against something in a way that if it were removed he would fall.

6.      Loss of consciousness.

7.      Madness.

8.      Drunkedness.

9.      The laughter of an aware adult person a prayer that contains bowing and prostration, even for the one praying by head motions alone. If the laughter occurs after the tashahhud, the ablution is nullified, though not the prayer.

10.  (Direct contact between an erect penis and the vagina of a desirable woman.

 The ablution is not nullified by:

1.      Touching ones private parts.

2.      Touching a woman

Walaikum assalam,