The HSQL documentation has the following to say about the "NOT BLOCKING" option on backups:
Hot backup is performed if NOT BLOCKING is specified. In this mode, the database can be used during backup. This mode should only be used with very large databases. A hot backup set is less compact and takes longer to restore and use than a normal backup set produced with the BLOCKING option.
Our databases can be quite large, so we're definitely interested in not blocking the application during backup. Based on this description, it sounds like it should be "safe" to use NOT BLOCKING. My main concern is that it would backup "partial" transactions if changes were being made during backup.
Since there is no mention of data integrity issues, can I assume it's safe to use NOT BLOCKING if I'm not concerned about backup size or restore time?