If you are talking about specific blood magic spells, a la blood boiling, then I would agree that you would have to practice a spell like that. Simple amplification I don't believe requires practice.
For example, during the attack on the Divine, the Senior Enchanter Jeannot used a blood magic laced fire spell against Evangeline without any seeming prior knowledge how to amplify his spells.
Look:
1) Anything first requires practice, even walking.
2) Establishing the link between the mage and the blood is a feat that requires practice and even a some mana usage, though a lot less than using mana to power spells. It's like using a lever, you use less power but gain more.
3) Amplifying spells is very easy - you use more resource and the spell becomes more powerful, just like using muscles, more strain more power.