A rat would easily nibble into a blue barrel if it wanted to.
How far are your blue barrels from your Pt , what kitchen waste are you composting .
I use pallets for composting and haven't yet had a problem with rats ,(and we have three takeaways within
50 yards .
I can't think of a physical way to stop them as they would chew through most things , and I don't know of any safe chemical deterrants.