A Short Bit On Blood-Warm

I first came upon the term blood-warm (sometimes blood heat) as a temperature indicator while reading through Hannah Glasse's 1747 book, The Art of Cookery Made Plain and Simple.

It came up in a recipe for a pretty cake to describe the state of heated cream and butter to be used. Instantly captivated by the morbid weirdness of it to my modern mind, more than that, I found it remarkable how intuitive it was. No need for a thermometer to tell me 98.6°, I really could feel it. 

These days we use lukewarm. Interestingly, the word lukewarm dates to at least the 14th century, so why use blood-warm or blood heat I'm not sure. Cursory research tells me nothing—literally nothing—as it pertains to the term and cooking. That said, there must be something on this history out there. If you know, please enlighten me!  

As a culinary tool, blood-warm and blood heat seemed to loose favor as an adjective in the early 20th century, so I was beyond surprised to stumble across this book published in 1988 that uses blood heat copiously in its recipes: 

Anyone familiar with more European-centric baking books and their use of this term?