The story
From "blood 'n' 'oun's", from "blood and wounds". Several sources, including Harrison Garman's 1892 A Synopsis of the Reptiles and Amphibians of Illinois, say the frog is so called because "its peculiar bass notes […] have a fancied resemblance to the expression" blood and wounds and other similar-sounding and similarly imposing phrases like "be drowned".
Etymology from Wiktionary, CC BY-SA 4.0.