JPEG format normally compresses pictures in 8x8 blocks, and these blocks are aligned so the first one starts at top-left corner of the image. I think these leaks you’re talking about are limited to the inside of the 8x8 blocks.
So, if you want to redact a rectangle between pixels [ 12, 4 ] – [ 34, 18 ], redact [ 8, 0 ] – [ 40, 24 ] instead, this way the black rectangle will cover complete blocks.
So, if you want to redact a rectangle between pixels [ 12, 4 ] – [ 34, 18 ], redact [ 8, 0 ] – [ 40, 24 ] instead, this way the black rectangle will cover complete blocks.