Hey,

just noticed a couple of games in my online library with a blue dot on their mark. I presume this means they have been updated? Some of the titles included really surprise me so hence the question

Yes, it is supposed to mark an update. Note that (although it's often the case) this doesn't necessarily mean that the games themselves were updated; it could be just an internal change to the installer structure. You can get more information in the changelogs (if there are any) at the download section, the game-specific subforums and the What did just update? thread.

Yep, those blue dots do refer to updates.

ConsulCaesar is right. However, it doesn't mean with the absence of a blue dot that the game isn't updated. Recently, GOG has been a bit wonky on that notification front.

Yes, do be aware it’s all very hit and miss. Lots got flagged but didn’t have updates. Lots don’t have change logs, or the change log doesn’t say much. You can’t download old versions, so you would be stuck with a broken copy if you don’t have backups:
https://www.gog.com/forum/general/provide_a_full_and_complete_changelogged_download_system