Carb digestion has more to do with AMY1 numbers than blood type – more AMY1 copies means more carb digestive enzymes in your saliva.
The number of AMY1 copies is closely tied to genetics and ancestry… and so is blood type… so there may be a link to blood type as well, but it all comes down to your ability to digest carbs, which is all due to the number of AMY1 copies and amylase in your saliva.