Buckwheat flour is alright during stage one of the diet. Buckwheat is a seed of the plant family polygonaceae whereas grains such as wheat come from the poaceae or grass family. Grains, meaning grass seed should not be eaten during the Candida treatment.
One thing you can do is use it in the coconut bread; 1/2 of each flour. It’ll make a grainier more wholesome flavored bread. It is completely safe and is one of those wonder foods you can have while on the diet.