Light flickers a bit when I turn on an hand blender. What can be the case?
Likely, the LED power supply picks up some of the electrical noise (radio frequency interference or RFI) generated by the blender motor, and it "tricks" the regulator in the supply to cause the flicker.

If the flicker is not annoying, then there is no reason to do anything, but you can add RFI filtering to the blender and/or the LED lamp. The filter can be a simple plug adapter, or perhaps winding wires around ferrite beads. (The links are given to show sample devices, not necessarily recommendations.) Depending on proximity, frequency etc., both might be needed.