I had a similar situation in that I wanted to donate to my husband but I was too heavy. They did part of the testing, but would not do the balance of it until I lost weight. It took me nine months to lose all the weight I needed to lose to be acceptable. I had to start eating better, watched portion sizes and joined a gym for the first time in my life. My blood pressure was not bad, but it did improve with the weight loss as did other numbers.
As Fr Pat has said, you are still relatively young and they don't want the donation to give you major problems on into the future. And actually, high blood pressure is the second leading cause of kidney failure.
You may want to talk with your own physician to see what they say about the blood pressure and weight loss as they may be able to provide some guidance and help.