So the blockchain.info only records transactions that are in already solved blocks.
Normally miners share transactions and pass them around to each other, so that which ever miner solves the next block, those transactions will be included.
But it doesn't have to be that way. A miner could decide to not share his transactions with other miners, thus if the other miners solve the next block, the withheld transactions won't be that block.
When I send a transaction from blockchain.info, you're saying it's not getting propagated to all the nodes on the network and put in their mempool?
The attack is applied for transactions from customers of the cartel, not for transactions from customers of blockchain.info website although they are affected eventually as explained below.
If you are a very widespread cartel such as Amazon.com, joined with McDonalds, WalMart, etc, then you may control a majority of transactions. They could chose to not propagate them to other miners as I stated.
What is the benefit? Well for one motivation, stomping their competition. If their competition will gradually lose hashrate because other miners go bankrupt, then everyone who doesn't send through the cartel ends up with very slow confirmations, eventually hours and days as the cartel gains more and more control.