Usually two items will make a blower carb:
1) 1:1 linkage between the primaries and secondaries,
2) the metering block is modified in such a way that the power valve requires that a vacuum line be hooked up from a source below the blower (intake manifold). Normally a carb gets it's vacuum souce from the base plate, but because a carb on a blower is under constant vacuum the power valve would never open. With an external source the power valve is "pushed" open under boost.