No. For one, even signaling skip blocks costs bits, especially in video formats without arithmetic coding (everything but H.264). Usually it's one bit per 16x16 block of pixels.
Secondly, it completely breaks off-frame motion vectors, increasing the bit cost any time that an object or part of the scene moves in from outside the frame.
Third, if the black borders aren't on mod-16 boundaries, that further worsens compression on the edges.
Fourth, it breaks motion vector prediction for the top row of the frame in basically every video format newer than MPEG-2.
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