I've got some more info on the MessagesBlastDoorService process that has been consuming large amounts of memory in my MacBook Pro.
I use the iStat Menus utility to monitor activity and resource usage on my Mac. Today I saw that the PhotoLibraryD process was running (this goes through the Photos library and indexes characteristics about images for searching). It was using a good amount of CPU time and about 2.6 GB of memory. Not normally an issue because the Mac OS will run this process when the Photos app is idle. I decided to launch Activity Monitor and see if MessagesBlastDoorService was active. It was, and it was using the same about of memory as PhotoLibraryD. These were the only two processes using such large, and similar, amounts of memory. Could one process have two different names? Did Activity Monitor see the same process differently from iStat Menus?
I checked back later when PhotoLibraryD had become idle. I saw MessagesBlastDoorService was still in visible in Activity Monitor, and PhotoLibraryD was still visible in iStat Menus, holding almost 3 GB of memory. I force quit MessagesBlastDoorService in Activity Monitor and saw that PhotoLibraryD had also disappeared from iStat Menus.
The only conclusion I can draw from this is that when PhotoLibraryD has completed its scan of the photo library, it doesn't terminate and release the memory it's holding. I have no idea why, nor why one process seems to have two names when seen in different apps. If anyone can throw light on this, I'd like to know.
