Blessed on food and remembered it's a fast day

What is the ruling on a public fast when a person recites the first blessing on food and then remembers it's a fast day?

Even though the Mishnah Berurah wrote to taste, the custom is not to taste on a fast day even if one has blessed.

Shaarei Teshuva, Siman 568, SK 1 "ABHT" and "BRI" in the name of "Nechpa Bekesef" that one who was fasting and took some food and blessed it and remembered the fast should eat less than a kezayit because of the blessing, and the fast counts for him. In "Admat Kodesh" there is doubt about this, and his words do not seem correct, and also Mahar"SH Germizan wrote that one should do so. (And it is necessary to examine in the four fasts where it seems forbidden, he should say immediately when he remembers in "SHKM"). And what he objected that it is a forbidden thing, and in Siman 196, Seif 1 it is explained that this is not a blessing but a curse, it can be explained that they did not establish the obligation of the fast at the time of the blessing, since there is a Torah prohibition of "do not take", and therefore it is not considered a forbidden thing. And also that we do not find that eating a permitted thing at a time of prohibition is considered a curse. And it is not similar to eating a forbidden thing. In practice, my teacher the Rav instructed me that the custom is like "Admat Kodesh" not to eat.