Transporter, one covertly shifting black blocks (8,4)
I believe the answer is:
conveyor belt
'transporter' is the definition.
(I know that transporter can be written as conveyor belt)
'one covertly shifting black blocks' is the wordplay.
'shifting' indicates anagramming the letters.
'black' becomes 'b' (abbreviation as in pencil hardness).
'blocks' is an insertion indicator (the inside letters form a 'blockage').
'one'+'covertly'='onecovertly'
'onecovertly' is an anagram of 'conveyorelt'.
'conveyorelt' going around 'b' is 'CONVEYOR-BELT'.
(Other definitions for conveyor belt that I've seen before include "Transporter" , "Mechanism for moving things along" .)