Dell 1350cnw laser printer prints blotchy 100% black

Looking at the sample, it seems like the yellow and cyan are also a little splotchy, but much less so than the black. These are built such that you can't easily replace the drums, as they're buried inside the printer. The problem is that this is a Xerox engine, so it will be next to impossible to find the parts needed (thankfully, the only Xerox engine to do this). While it can be repaired with new drums, your ability to do it is based on part availability. The lower end Xeroxes tend to be hard to find parts for. Xerox still sells the toner but doesn't consider the drum/sev serviceable, likely because of how Xerox built it. The first thing I'd try on a unit like this is to replace the problem toners and pray this fixes the issue. And that's if the fuser temp adjustment doesn't help you here or it's buried in the SM.

I'm not even kidding - you WILL NOT find these drum/dev units readily being from a Xerox engine, especially since you likely won't be able to recalibrate the printer to take the replacements correctly. This is based on the Phaser 6022, best I can tell. But I also know Kyocera uses a engine with similar toners - so maybe it's a Kyocera engine Xerox rebadged given their ECOSYS color printers have a similar drum/dev format?

I quickly searched for the drum units for the 6022 (the closest match I can see; the ECOSYS printers use a similar engine with color models where the drums aren't readily replaceable), and I haven't found anything on it. In this case, you'll need to find a donor printer with some other fatal issue that works well enough to confirm the drums are good and swap them out that way - and hope you can do the calibration and sort it.

I say this because when you do a big job like swapping a sealed developer unit, the "user accessible" option usually doesn't cut it - you need to do it in the SM to get it correct. What gave away this being Xerox is the "BTR" adjustment - that's 100% a Xerox thing.