Is the neutral/blank livery supposed to be choosable at certain airports only? I cannot choose it at EDDK nor EDDL.
If with "choosable"; you mean always be available everywhere automatically in the operator selection menu, then no, it isn't and it would be wrong if it were, because it would mean giving it the highest priority in the scoring system even with we have proper information about which operator is really there.
In fact, it's exactly the opposite, and it has the lowest possible priority possible, precisely to prevent it to appear everywhere, but still be available in the parking customization, which means if you find an airport where you are absolutely sure we assigned the wrong operators, and you can't find any suitable one either, you can assign it the blank one.