VISA work in all country, but a lot country not allow their people to open account outside domicil country. If I understude you plan to open account lets say in Ukraina for man from Nigeria with their personal document, but Nigeria law say their people can own just account in Nigeria. Its not important where you open account, they cant own account outside Nigeria. Nigeria is just example, there is more country...use Google to check
If it is seriously blockchain based it shouldn't matter. If it's a dirty centralized service making use of blockchain, these things will fail anyway.
The fact is, VISA is centralized. Fiat system is centralized. There's always a central bank governing fiat. And we can't just tell them to be decentralized. Only the application matters and how this two systems interact. We pay in Crypto, there's always a process of exchange happening. If merchant accepts crypto, then there should be no problem. If one accepts fiat then there should be no problem too as long as the process went well. Law of Equivalent exchange is what matters.