Well, I can understand where you are coming from--transfusion issues can potentially involve a lot of conflict with clinicians. Conflict and confrontation doesn't bother me terribly, but I've seen peers who seriously would rather do just about anything than get into an argument. Surgeons in particular can be rough for some, because they are trained to be very "assertive", and they really don't like hearing "no".
Some suggestions I can make to help with this:
1. Particularly during the day, if there is a call that is a potential source of conflict, go to the site of the problem. A bleeder in the OR? Get in there. Some team on the floor insisting on getting the INR down to 1.4 before dropping a line and flooding some cirrhotic patient with liters of FFP to do that? Go face to face with the resident/fellow to talk about it. One of the biggest complaints I've heard from clinicans about pathologists is that they manage from their office or home, and don't care enough to show up to discuss things. Believe me when I say that they will respect you more (and more importantly, listen more to what you say) when you are saying it to their face than to a phone speaker.
Even if you are still learning about how to manage a particular patient, and you feel you don't have a lot to contribute, just showing up with a "how can I help you" attitude will go far in diffusing tensions. Make it clear in every call you handle that you both share the same concern: getting the best care possible to their patient.
2. If I get a questionable order (i.e. someone ordering 12 units of FFP for something that doesn't make sense), and it sounds like the case has potential to be time sensitive, I'll send along a small portion of the order (i.e. 3 units) right away, then call to sort things out. I'll start the conversation with "Hi, heard about the issue, 3 units are on the way, but can you fill me in on what's going on?" This starts things off on a cooperative note, and they will be more inclined to agree when you say "I think this may not be necessary, and here's why".
3. Sometimes you are just going to run into some pissant resident/fellow who is trying to assert their authority. At this point in the game, you have to reconcile yourself that some discussions have to occur between attendings. Know when to throw in the towel and call in the cavalry. But ask your attending to listen to them, and how they handle it.
4. It goes without saying that if you know your facts, you will be much better armed to argue with someone if it comes to that. Know indications of blood products, and better than that, know the literature *why* those indications are in place.
I know it probably doesn't help you much to say this, but conflict is going to happen in your career/life, period. It really is good for you to learn to deal with this and not run away by dropping CP (and shooting yourself in the foot careerwise to boot) . Perhaps picking up a book on conflict resolution (Try "Getting to Yes" by Fisher and Ury to start) will be helpful. Look at blood bank call as a good practice for developing these skills. Realize that "soft skills" like negotiation and conflict resolution actually will make a big impact on your career in the future. Take two candidates with roughly equal skills, and its a good bet that the person who gets along well, can negotiate contracts, and resolve issues between individuals and groups will have swifter and higher career advancement.
Good luck, and don't give up yet.