I think it is wrong to call bitcoin a "private" currency.
On the contrary, imo bitcoin is much more of a public currency that any national currency is.
Bitcoin is not a private currency but using of bitcoin is private for sure. Since its anonymous we can send or receive the a lot of money without giving any data or identity. Bitcoin is a public currency even AtMs are build in some countries many hostel schools are accepting it there's no chance of calling bitcoins as a private currency. Maybe you're calling it private because they're not exist physically but its internet currency and everyone is welcome to use it