If there is a pitot tube blocked the radar controller will not give you a different altitude than your altimeter, except on 1013 than local QNH. Aero Peru 757 Flight 603 relied on the same misunderstanding and touched the ocean's surface. Ground speed may help a little to judge IAS, though.
What does a blocked pitot tube have to do with erroneous altitude indications???
I have had blocked pitots before but never any issue with altitude readouts.
And wasn't the issue in the Aero Peru accident blocked static ports?
Beech