Why do you new to cover the controls?
Our blech is a simulation of the “grufa uk’tumah” of the Mishnah. The reminder not to raise the fire was evident in both the fire itself and in the “controls” (the fire was stoked, so the fire was both the heat source and the control). In our stoves, the control knobs are apart from the fire. If a reminder needs to be evident in the controls as well (as was the case in the times of the Mishnah), the knobs need to be covered. (Based on IG”M O.C. 1: 93)
Truth be told according to the Rema one could return something to the blech b’dieved without either of the conditions, as long as it is not a liquid item that has cooled off completely.
He says, though, “tov l’hachmir”, and based on M”B’s explanation, his heter might not apply to modern stoves.
Why not, if it’s solid? Ein Bishul Achar Bishul.
Because of “nireh k’mevashel” (see S.A. Horav 253: 14, and IG”M O.C. 1: 93 that a blech does not help for this).