Do you have a reference you're working towards? Like most uniform colours there was a fair bit of variation and if you have a specific target in mind there are good and bad choices to work with.
In general for darkish blues I like to recommend starting with any version of French Ultramarine because it will help ensure matt drying. Add a little Burnt Umber* to dull it down, then mix in as much white as needed. This is a good basic mix for blue uniforms and can be tweaked as needed in any direction you want - more greenish, more violet, more vivid, each only requires a touch of one additional colour.
*Including some umber also helps with a matt finish and should aid drying if you're not using any kind of heating to speed this up.
Einion