Wow, what a great little house! Any interior shots? I can't remember if I had seen any. I am particularly fond of Foursquares since I live in one myself.
Colour suggestions: Generally, older houses were not blue since blue paints tended to fade. If you are not concerned with historical accuracy, and you just want ideas for a nice, historic looking colour, I'd suggest a mustard-gold, light green, pink, light yellow. Any of these would look nice. There's also nothing wrong with the current blue.
For the trim, if you want the house to look crisp and fresh, keep it white, but if you want it to look a bit older (more vintage), consider an off-white. Others have suggested highlighted trim, but I find that the effect looks better on fancier Victorians than on a simple Foursquare. It's up to you. If you do want to highlight the details, I'd aim for a subtle difference, like a cream for the details (over an overall white background).
Side note: Do you have a t-g porch ceiling? If so, and if it's already been painted (white I'm assuming), you might consider repainting it a medium brown to fake the effect of the original stained wood. If it's already stained wood, congrats!