Blow vs suck air over hotend's heatsink
Getting hot air from the radiator is definitely wrong idea because of few reasons:


hot air can damage your fan (as they are usually not heatproof). Cold air cools the fan
cold air is denser so fan can suck more cold air than hot air so cooling is more efficient (fig A)
in terms of plug of radiator (with dust) it's much better to try to push cold air into plugged fan (fig C) than to suck the air from it (fig B). When flow is decreased there is higher possibility to damage the fan as it doesn't cool itself


Of course we are talking about 3D printers with small plastic fans. Please notice (almost) all fans construction: they are designed to get air from the top of the fan and push it to the bottom (this construction prevents damaging/plugging fan motor by dust)... and their mounting wholes (or clips) are usually... naturally suggesting to direct air to the heatsink)