Already answered this here
If two institutions have agreed to trade at a certain price, they can do it in this separate window. If they try to buy/sell on the normal market it will be hard to execute a deal like this.
You can track the block deal activity here on NSE.
https://www.nseindia.com/market-data/block-deal-watch
Check this FAQ if you want.
Big fishes trade on the normal markets itself.
trade on the normal markets itself.