If you want one to flower each year you need a perennial not an annual. A photo of the bed would help a lot to give you good advice.

I'd say go for something with strong foliage that will hide the bluebell leaves once the flowers have gone.
My go-to plant - hostas. The foliage will, generally, start to come up as the bluebells are flowering and will be well-grown by the time the bluebell's are looking less than perfect. Depending on the size of the fountain, I'd probably go for some big varieties such as First Frost (also later flowering), Wide Brim and Blue Mammoth.
A picture of the bed and fountain (with some idea of the size) would be helpful.

A photo would be good, but we also need to know whether the area is very sunny or very shady. If you know which way it faces (north, west, whatever) that would be good to know too, and what part of the country you're in - some plants are hardier than others, so there's a difference between, say, living in Scotland or the North, and the West Country or down south.

You may find this article helpful http://www.telegraph.co.uk/gardening/howtogrow/7686817/How-to-grow-native-bluebells.html

It suggests some companion planting:
Dryopteris filix-mas 'Crispa Cristata' and 'Grandiceps Wills'
Himalayan forms of Dryopteris wallichiana (Wallich's fern)
Milium effusum 'Aureum'