If you have Priests, why use Blossom Mages at all? Blossoms are the only game in town for an elven player. But if you have Priests then you're playing as a human.
You are right about 3*** Priests, they are much better than Blossoms specially against HRs. Priests deliver heavier punch against HRs as Blossom's main specialty is dealing with HMs.
I use Blossoms to shoot enemy Thronerose mages when the terrain is blocking the LR movement but two step mages are ok.
The Thronerose has higher initiative and takes the first shot at my Prists, causing them weakness and making them less effective. So I use Blossoms against Throneroses when I can't use LRs as Blossom's higher initiative allows me to shoot the Throneroses first, hopefully to kill it or drastically reduce it's power.
I'm in chapter 9, still a few chapters away to have a 3*** Blossom, but I still I prefer Priests when I don't have to deal with Thonerose mages. Sometimes I use Banshees if range is not an issue and I know my mage would get hit by HRs anyway.