There is no blockchain balance. Blockchain is a technology system, not related to economy. Bitcoin is a payment made to promote blockchain technology. You can convert
Bitcoin to cash on exchange through the exchange, you can find a seller from a country you are on
https://localbitcoins.com.
Im also saying the same thing, if you want to withdraw your Bitcoin you should make use of localbitcoins to find an exchanger and then sell to them. But I think the op knows it is called Bitcoin, cause from what he said how do I withdraw my blockchain balance, so theres no need to think hes a dummy.
Blockchain does not provide that service as of now. If you want to buy and sell you should have the wallet in the coinbase wallet. You can do this expectation with that wallet still they are not a trusted wallet by anyone since they does not shared the private key to us.
As you said there are some fiat exchange providers are there in the market, LBC is the one of the best service provider to exchange the bitcoin to cash. Almost for all the countries they give the service. If they takeaway the deposit fees as like before. It would be the best exchange.