I think it was the blockchain […] can they charge a commission?
Small b blockchain
The word blockchain, with a small “b”, usually refers to the list of all Bitcoin transactions worldwide since the beginning. This is a chain of blocks (like a book of pages). Every full Bitcoin node keeps a copy of this list called the blockchain, including wallets using software such as Bitcoin core.
Since this list is not a person a business or any type of organisation it can’t do things like charge a commission.
When you use a wallet to spend money, your wallet gathers enough amounts of money as “inputs” to make the desired payment “output”. Your wallet reserves an additional amount you select called the transaction fee. The transaction fee pays the miners. Note that miners can’t take money or make charges, they can only collect the fee you have set when you create your transaction. If there is any money left over from the
inputs - output - transaction-fee, your wallet specifies an extra output that returns this “change” back to you.
Note that you choose the amount of the transaction fee. A small fee means it takes longer for miners to run out of better paying transactions to include in new blocks and get around to including yours. No one can alter your transaction to add a commission charge.
Note also it isn’t possible for the recipient to only receive a part of the amount you specified as the output amount to send them. They get the whole amount when the transaction is confirmed or they get nothing if the fee is so low that the transaction never gets confirmed – in that case, you still have the money.
Big B Blockchain
There is at least one business that chose to include the word blockchain in their name. This is Blockchain.com, Inc. They have had other corporate names in the past. They are not in charge of Bitcoin and they are not in charge of the blockchain. Lots of Bitcoin users never use this business. Nor any other business like it.
Bitcoin was designed to be used without the involvement of this type of business but many people choose to give their money to this sort of business to look after for them.
This business can charge you whatever fees they want, the legal jurisdiction in which you live probably requires that any such fees are described in the terms and conditions document that the business makes available to its customers.
Swindlers and confidence tricksters like to persuade their victims that extra fees need to be paid. Unexpected fees are a common feature of this kind of fraud.