Your colour choice sounds promising. There's a lovely rich blue Salvia which is in flower now, but most plants are coming to the end of their flowering season. Nemesia is a good long-flowering, low growing plant and there are many varieties in blue, lilac and white.

There are some nice blue veronicas too, and light blue penstemons that flower for a long time. Heather will not have a very long flowering season. Do you mean the summer one or the winter Erica? The summer one needs an acid soil so that might be a consideration and it only flowers late summer. Penstemons again would give you pink and/or white for a long time. Heucheras come in many colours and would give you interest though the winter as well as the rest of the year. If the bed is sunny you would need the darker ones. Armeria (thrift) is a nice pink for the front of the border. Old fashioned pinks flower for a long period and have silvery leaves.

How about a dwarf azer in middle them plant dwarf spring bulbs around around

For summer plant up geranium summer bedding.

Good luck.

How about a bit of height with a couple of tall allium or verbena bonensis to break up the profile - I know it's a raised bed but that doesn't mean you should only have low growing plants. Verbena is very airy and will waft about while allium, especially the big flowered ones make a good strong architectural statement. One of the October offers in Gardeners World magazine was 100 allium Modern Art for £3.99

Have you considered Heucheras would give all year round mixed colours and flower all summer.

I've got a question - how much sun does the bed get?