As Blood DK i need to know how much damage i have taken the last 5 second to understand how much my Death Strike heals me for.

There is Weak Auras that have this implemented already so its should be simple to re use.

https://wago.io/NkaTBpcPW

With the changes to blood with shadowlands it's more important then ever to death strike wisely. It would be nice there was a clean function to get how much it would heal for.

Indeed that is a nice idea, will look into adding it using events tracker.