Head gaskets can/will blow. If there are marks on the TOP of your piston, as you said, that means there was metal floating around in the combustion chamber long enough to cause some damage. How does your compression feel? Significantly lower than before?
The usual cause of blown/burnt gaskets after an assembly is improper or uneven torque on the fasteners. It's likely you had an air leak long before you popped that gasket.
How did your plug look? Was the jetting correct?