Hi! It sounds like you've got a plan of action in place already, but I thought I'd share some information in case it's helpful to you. I have pretty bad allergies too, so I've tried a variety of over-the-counter medications. I haven't had any adverse reactions with blood pressure. A primary care doctor basically told me that in general most medications are safe for a kidney donor, but I should be cautious of medications that get metabolized in the kidney because long-term use can be stressful for the kidney. High blood pressure could be a separate side effect and as I understand it it's not uncommon for antihistamines.
But if you're concerned, I think you're doing the right thing by talking to the coordinator at your transplant center. Or, you may want to consult with a pharmacist--my mom (my kidney recipient) has done this for general information and feedback with success. Also, she had a history of high blood pressure (then low blood pressure after going on her transplant meds) so she monitors her blood pressure regularly to look for spikes and dips over the course of several days. Her doctor was able to work with her on it after they could identify some type of documented pattern.
Good luck, hope you're feeling better!