There aren't many (if any) electronics faults that can cause this, thankfully...
First press the test button on the side of the bottom casing round the back. Does the test printout also have this strip missing? If not, then you've got a very strange problem with the formatter board, and I'll have to think again...
Secondly, take out the cartridge and shake it, and put it back in. If that doesn't help, then try a new toner cartridge.
If you've still got problems, then the most likely problem is something in the beam. SX engines (which you have, and CX engines and...) print black where the drum is discharged by the laser . Start by removing the toner cartridge. There's a black metal strip in the top cover above the toner cartridge. By fiddling with it you can move it slightly, exposing a front-silvered mirror. Inspect that for bits of paper, etc. Clean it _very_ carefully.
Next area to look at is the laser scanner itself. If you know what you are doing, take off the outer casing (screws round the outside, some under the lip that the lid closes onto, and 2 on a metal bracket at the left side of the fuser) and look at the scanner block. This is the black thing mounted over the paper tray. Unplug the cables (one to the laser PCB itself, one to the scanner motor under the little flap on top), undo the small screw to release the optical fiber, and undo the 4 screws that hold the scanner in place. Lift it out. Look at the slot on the front edge for any paper, etc. I don't know if I recommend you opening the scanner (Canon DO NOT, but I've done it), but you can carefully trim back the heat-moulded studs and open the cover. Look for debris inside. Handle it with great care, of course.
(From: Ken Eckert (firstname.lastname@example.org).)
One problem that gives this printout is loose screws on a plate in the top cover above the toner cartridge. The screw(s) fall out and get stuck in the toner shutter window that that the laser goes through. I've had a couple like that, I made it a habit to tighten those screws whenever I work on one of these Canon engines.