You've probably been given spanish bluebells rather than English ones - these do come in pink, white or blue, and often will produce offset bulbs which may not be the same colour as the parent plant. It is odd that all the white ones are in one border and the blue in the other, but they are not sensitive to soil ph in terms of flower colour, so it could just be fluke that this has happened. I can't think of another explanation.

as bamboo says they are not pH sensitive for flower colour. Is one side shaded and the other in full sun? bleaching in strong sun may make blue fade and appear almost white.

welcome to GoY too :o)

Thanks for the replies. However, the blue ones are on the side that is probably sunnier. And the fact that ALL are Blue on one side and ALL are white on the other means that it would need to be a very big fluke. The soil is slightly different - clay on the white side and better soil on the blue. Could there be another reason related to the soil besides the ph? I am sorry to be so ignorant. But it would be good to know if there is a difference in what other plants I can grow. Thanks.

Well, other than doing soil sample testing, there's no way to know unforunately. Soil acidity or alkalinity (ph) does make a difference to some plants, but bluebells aren't one of them.