Replacing an entire keyboard on a MacBook (Air or not) is a hard operation or a delicate operation if you just do the key caps. It is in the realm of possible, but almost never is it practical. The systems are designed to have the top case swapped out and over time, the battery is also becoming part of the top case in some cases as are the keyboard caps. Where they require delicate tools and glue / adhesive skills to replace parts other than the whole top case.
In every case I've seen, there is at least one difference (typically a handful of differences on the 82 or so keys) in the key cap physical size between a US model and a european model. Total replacement of keycaps isn't a viable option.
Apple doesn't sell key caps to end users, so you will want to look up a Service Provider to ask them what they would charge you to sell you the black replacement caps if you ordered a set for the US model of your Mac.
You'll also ask them how much the scissors underneath would cost (and how long parts take to arrive) in case you break the delicate plastic parts while removing the caps. My guess is you'll get many of the keys replaced easily for not a whole lot of money, but the special keys will be shaped in a way that you can't replace them.
Source: How to identify keyboard localizations