It does blow warmish on the passenger side as long as you don't blast the heat. More indications that it is a plugged core IMO.Also, if it blew heat on the test drive as Spork says, why would that change now? Do you think the antifreeze mix if somehow off? Were both driver and passenger sides checked?
It'll be interesting to drain and flush the core. Perhaps the previous owner had the coolant replaced with the incorrect type? I'm not sure what I'll find. The overflow tank has blue coolant in it, but that doesn't mean there isn't something else in the block. I'll be draining the whole thing and probably flushing the radiator, block, and heater core depending on what I all find.