I hope to learn about blood: Does it invalidate prayer?

There is no legal (Shari'ah) evidence to prove that bleeding from other than the private parts invalidates ablution (wudu'). The default is that it does not. Although some scholars believe that it invalidates ablution if it is profuse, performing ablution from it as a precaution and to avoid disagreement is good, due to the Prophet's (peace be upon him) saying: "Leave what makes you doubt for what does not make you doubt."