I have heard people say that somebody "has blood sugar" when they mean "has diabetes". Today, that sounds ignorant; two hundred years ago it would have been scientific opinion.
I have not been able to tease out which sources are accurate with regards to who did what in terms of figuring out that in general blood DOES contain sugar. However, the following people seem to have been involved:
Hermann von Fehling (1812- 1885) a German chemist, after whom Fehling's solution is named. He lived in what is now Germany, and he studied chemistry. His solution is used to test for the presence of sugar, and it was still in use as the primary test for sugar when insulin came around. He developed it in 1848, and appears to have been aided in part by
Barreswill 1817-1870 a photographer, who was working on developing chemicals for better colors in pictures; finding out what chemicals changed color in reaction to what was useful to Fehling, who may have been a friend of his.
Claude Bernard (1813-1878) whose experiments on animals demonstrated that the liver releases sugars in the blood stream even in animals that haven't been consuming sugar, and that beyond a shadow of a doubt, sugar in the blood is normal. His main experiments with the pancreas were with digestive juices; he suspected diabetes was caused by signaling issues between the liver and brain.