The rule with blessings is, 1) If a fruit or vegetable was pureed but not completely, one says the relevant blessing to that fruit or vegetable – ‘Ho’ etz’ or ‘Ho’Adomo’. 2) If the fruit or vegetable was completely pureed so that there are no recognizable pieces, one makes the blessing ‘Shehakol Nihya biDvaro’. (Seder Birchos Hanehenin 7:22)
There are two ways to make corn flakes, 1) The corn is ground completely to cornflour, in which case the blessing is ‘Shehakol Nihya biDvaro’. 2) The corn is not ground completely and there are still pieces, in this case the blessing is ‘Borei Pri Ho’Adomo’.
For example, in Eretz Yisroel the companie ‘B & D’, and ‘Kellogs’ company overseas, manufacture cornflakes where the corn has not been ground completely. The blessing is ‘Borei Pri Ho’Adomo’.
On the other hand, the Israeli company ‘Telma’ and the overseas company ‘General Mills’, manufacture the flakes from corn which has been ground entirely to flour and thus the blessing is ‘Shehakol Nihya biDvaro’.
If it is not known in which manner the flakes were produced, one should say the blessing ‘Shehakol Nihye biDvaro’.