Block trades are large volume transactions to buy/sell an asset without spooking the market. Block trades are not disclosed to other market participants until execution is completed because disclosing it prior to execution can cause a stampede due to herd mentality.
Example of a Block Trade
A companys stock may be trading at $20 per share but for some reasons they agree to sell 100,000 units of the shares to an investor for $19 per share. Such a transaction is a block trade & will be privately negotiated and executed away from the public eye so as not to drive the price of the stock down.