The episode title is a twist on the term "Black Widow", which in a criminological context refers to female killers who murder their husbands or boyfriends.
Psycho: When Sideshow Bob goes to see Selma's dead corpse in the chair, he turns it around to see that Bart is there instead. He whacks the light, and as it swings back and forth, he sees Selma in the doorway. This sequence parodies the film's famous ending scene.
Encyclopedia Brown: Bob's scheme (and its revealing) are similar to the pattern of how Encyclopedia Brown stories unfold.
When Patty tells the family that there is something disturbing about Selma's new boyfriend, Lisa imagines Selma walking arm in arm with a tall man in a big coat and hat with a bag over his head with one eye hole and a walking stick. This appears to be a reference to Joseph Merrick, The Elephant Man.
When Bob is saying goodbye to his fellow inmates he says goodbye to the first two and then turns to Snake to say that he thinks he is going to miss him most of all. This mirrors the goodbye scene from The Wizard of Oz.
When Sideshow Bob is arrested, he remarks that once the Democrats are back in power, he'll be back on the streets. Ironically, in "Sideshow Bob Roberts", Mayor Quimby, who is a Democrat, gives him a pardon after pressure from the Republicans.
This is not technically a goof, as Bob refers specifically to Democrats in the White House.
Bart, after putting the facts together, states that Selma has only an hour to live. The Simpson family is then able to rush to their honeymoon hotel and rescue Selma from the potential explosion threat. But Selma and Bob are vacationing outside of Springfield, making it unlikely they could have gotten there in the relatively short amount of time mentioned by Bart. (They were in Shelbyvile which was shown to be 34 miles away via the highway sign.)