i seem to remember when i was younger having this sort of...mini airbrush thing that you could pump air to with a thing that went under your foot....i cant remember what it was called, it was in some sort of set for something but that would have been so useful....
i suppose the only other way is to invest in an airbrush, Games Workshop do a fairly cheap one that you can use with compressed air canisters, im not sure how fine you could get detail with them but they are used for painting miniatures so they cant be that bad. i think they are about $20 if i remember rightly.
beyond that all i can suggest is really really watering down the paint and making a patch of water where you want the blush and putting a dot of the paint in the centre of it so it gently gradiates towards the edges....hard to explain but it might work