If you transplant a hydrangea when it's blooming, will you kill it? (I know, really stupid question!!!!!)
transplanting
It's not any more likely to kill it than transplanting when it's not blooming (transplanting always carries some risk). It may lose its blooms though, so if you want to enjoy the flowers I'd wait until it's done blooming. The middle of the summer isn't the best time to transplant things anyway, heat stress combined with transplanting stress decreases your chances of success.