In "Lamb to the Slaughter," does the line "...Noonan discovered a small patch of congealed blood on the dead man's head" imply that there was a chance, despite the destruction of evidence, that Mrs. Maloney could have been found guilty?

There was always the chance that the investigating officers would've picked up on the available clues, such as the spot of congealed blood, and drawn the conclusion that Mary killed her husband. Yet they chose not to do so. For one thing, they're taken in by Mary's incredibly convincing story. What's more, they're unable to see past her mousy, timid demeanor and so can't begin to imagine that they have a killer in their midst.
The detectives appear quite thorough in carrying out the proper procedure; they ask Mary lots of questions and try to establish her precise whereabouts at the time of the murder. But once Jack Noonan utters those magic words, "Get the weapon, and you've got the man," it's almost certain that Mary will literally get away with murder. The police have linked the murder weapon to the killer's identity, and because Mary has cleverly disposed of the evidence—by cooking it, no less—they're not about to establish a connection between her and the murder she committed.

This question makes an interesting point, and it is certainly possible that the police might discover Mrs. Maloney to be guilty of the murder of her husband based on evidence like this patch of blood. The line containing the description of the blood, however, may simply function as a strong visual image that forces the reader to face the reality of Mary Maloney's crime of passion.
The style and tone of this short story by Roald Dahl is characteristically matter-of-fact, which makes the imagery of "congealed blood" so powerful. Dahl chooses to write in a sparse manner, with few descriptors, relying instead on the power of an image, like the blood on Maloney's head, to convey an emotion like fear or horror.