Is it only available in Polish złoty for Polish citizens? Or could I, from a different EU country, open an account and use it to pay for games on Steam? My guess would be no, but it's worth asking.
It's supported by polish banks for PLN accounts, but I don't think there's a requirement to be a citizen to open an account. I know Revolut also allows using it, if you create a PLN account in it.
Revolut will only offer BLIK if your address is in Poland, it's not enough to just open a PLN subaccount. Same with Wero, it is only shown for customers in Germany and France IIRC. I have no idea why they did it like that, the backend clearly supports them all.
PSP S.A. is expanding abroad, so perhaps BLIK will be available in more countries in some time: "In September, the first BLIK transaction was made in Slovakia, and BLIK Romania S.A. received authorization from the National Bank of Romania to operate in the country. In November, the Polish Payments Standard transformed into a joint stock company, to support the execution of its strategic goals." [1] [2]
BLIK has one ugly caveat, though: it has no chargeback procedure.