I never bought a blockclock, but I do remember that a competitor once made something called a btclock and that it infuriated nvk.

well, now the makers of the btclock have an update that allows you to track the number of bitcoin stolen due to Coldcards.

source

They are also suggesting you look at a security analysis for Coinkite's Blockclock product which can be found here: https://gist.github.com/dsbaars/33ace96982773b5dc79d46551f64d467