Allen keys are magnetic (ferrous) and will get automatically sorted at the material recovery facility. So it is safe to put them in blue bins.
But because they are small, some general waste collectors may not collect them as they get lost / fall through the cracks during collection of segregated bins.
So blue bins is your best option.
Everyone asks: will Ikea take these back? Yes and no.
Ikea will not take these back for re-use. If you return them to Ikea, they will simply get sent for recycling as if you put them in the blue bin yourself. So save yourself a trip and recycle it yourself.