If you want to insert a new block but have its default be off in your snapshots, here's how to do it.
Insert the block as you normally would.
Hit the Action button and set the Snapshot Bypass parameter to Off (or right click on the block in HX Edit and select Snapshot Bypass from the menu that appears and set it to Off).
Toggle the bypass state of the block to Off.
Repeat Step 2, but turn the Snapshot Bypass parameter back to On.
Now the block will be Off in all your snapshots. Turn it on in the ones you want it on in.