Thanks for answering. If it is not safe to do so then I will not delete BLOBDIR for now. The reason why I asked, if it was not already obvious, is that I would like to create a lightweight backup that will allow me to recover my account and contacts at any time, but because the backup bundles the chat messages with the account and contacts data, the backup file size can be hugely bloated, especially if there is a lot of multimedia in the chats.
When I looked at the backup directory structure, the BLOBDIR directory seemed to contain only chat messages/media, and so I speculated that it might be safe to delete it, but I won’t do that if there is no guarantee of safety.
Hopefully the backup feature will in future make it optional to include the chat messages, so it will be possible to make lightweight backups with minimal file size.