Welcome, Kernowboy!
In deeper shade, the flowers become lighter blue, but white probably means that the plant has mutated at some point, and the shoots and flowers growing from the mutated cells are a different color. Breeders call that a "bud sport". Occasionally, plants and even individual flower heads can have both colors. If this is the first time the plant has bloomed since you got it, it may just be a mistake on the part of the garden center, or the grower they bought it from.

I'm so glad I read this question! I bought an agapanthus from a GC here early this year and when it finally flowered I found I had both blue and white flowers and thought I had two plants in the same pot - difficult to tell with all those roots! Will it stay bi-coloured or will it revert to one colour with time and TLC?

Usually, the original blue one is slightly more vigorous, so it is likely to take over--doesn't always happen, though.

Yes, I have noticed that the blue section seems stronger. I'll pin all my hope on the blue! ;o)