Blocks cannot simply be typed into the book without moving the price. They are negotiated upstairs, crossed in a dark-pool, or worked over time by an algorithmic-order.
Because a block is printed to the tape after it is agreed, a large print appearing well away from recent trading often marks a negotiated cross, not a sudden change of opinion.
Example: 400,000 shares of a stock that trades 1.2 million a day is a third of daily volume. Sent as a market-order it might cost several percent of market-impact; crossed at the midpoint-peg with a single counterparty it costs half a bid-ask-spread and a negotiated fee.
Related: not-held-order, crossing-network, request-for-quote