I cut an onion with a fleishig knife and put it into a parave blender.
Is the blender still parave?
There is controversy about this, and many poskim say that there is no need to kasher it. It is however a preference to kasher the actual blade, by inserting it into a pot of boiling water.
Magen Avrohom O:CH 451-31, Chochmas Adom 49-10, M:B 451-90, Even Haozer Y:D 96-3, Badei Hashulchan 96-56, Davar Chorif 4-2, HIlchis Basar Bchalav 10 ftnt. 5 from R’ N. Karelitz zt”l and R. E. Auerbach, and R’ Veber shlita.