'Blood and thunder' originated as an oath and, while not a specifically religious phrase, took its lead from the numerous euphemistic minced oaths, which refer to divine personae while avoiding the literal use of sacred names. 'Zounds' (God's wounds) and 'sacré bleu' (God's blood) come to mind in this context. Lord Byron confirms that view in the satirical poem Don Juan, 1818–24:
"Oh blood and thunder! and oh blood and wounds! These are but vulgar oaths."
The phrase is found as early as the mid 18th century in The Adventures of Peregrine Pickle by Tobias Smollett, 1751:
..."smiting the table with his fist, he started up, and, with the most violent emphasis of rage and indignation, exclaimed, "Darn my heart and liver! 'tis a land lie, d'ye see; and I will maintain it to be a lie, from the sprit-sail yard to the mizen-top-sail haulyards! Blood and thunder!"
In the same work, Smollett also used the phrase to refer to a person or type of person, by using 'blood and thunder' as a form of name:
"Blood and thunder! meaning me, sir?"
That usage was repeated later in an advert for a play, in The Times from November 1789 - just a year after the paper was first printed. It's quite appropriate to find an early usage of the phrase there as The Times is itself nicknamed The Thunderer. The entry publicised The Newspaper Coalition - 'a farce in two acts', and included reference to a character called 'Blood and Thunder' - a hunting parson (or parfon, as The Times then styled it).
'Blood and thunder' became a stock expression for authors of historical melodramas and by the 19th century such cheap 'penny dreadful' fictions were also known as 'blood and thunders'. Despite being scorned by the literary elite, in a similar way that sex and shopping and aga saga novels are scorned today, that champion of popular culture, G. K. Chesterton made a case for the inherent truthfulness of 'blood and thunder' romances:
"So long as the coarse and thin texture of mere current popular romance is not touched by a paltry culture it will never he vitally immoral. Their drivelling literature will always be a 'blood and thunder' literature, as simple as the thunder of heaven and the blood of men."
The spooneristic 'thud and blunder' was too good to miss and began independent life in the late 19th century. For example, a piece from the Kansas newspaper The Globe, April 1879, headed:
THUD AND BLUNDER, A Chapter of Highway Robberies, Fights and Thefts.