Gun blue can be applied with a foam applicator, it takes several applications to achieve consistency. Some steel wool in-between coats also helps with that.
Insta-blak is also another good option, it's a bit pricey though, so would be expensive to buy and totally immerse the parts. I've put it in a spray bottle and hung up leaf springs for application.
With either gun blue or insta-blak, they need to be rinsed with water, thoroughly dried and a rust inhibitor applied. The ECS product is the best I have found. It has to be something that leaves a film. Boeshield or cosmoline do not have the mil coverage to protect for long periods. Nothing worse than having to steel wool already assembled leaf springs that have surface rust everywhere!