Blocks are arranged between a client and a dealer, priced as a package, and reported afterwards. They frequently print outside the displayed nbbo because the true trade includes a stock leg or multiple options legs that the tape reports separately or not at all.
This is why blocks are the most misread prints in flow data. A 20,000-lot put purchase at a price through the bid looks alarming until you learn it was the protective leg of a collar on a position ten times the size.
Example: 15,000 XYZ $45 puts print at $0.92 when the screen shows $0.88 bid, $0.95 ask. Elsewhere, 15,000 $55 calls print at $0.80. One institution collared a 1.5 million share position, and neither print means anything on its own.
Related: options-flow, complex-order-book, collar, options-sweep