Black smear on underside of paper - Canon PIXMA MP830 All-In-One InkJet Printer

Sounds as though you have a leaking black cartridge and ink has got on to the platen.
Open the machine as if you replace a cartridge. Remove the whole print head unit. You can do it without taking the inks out. What does the underneath of the print unit look like? Pretty messy I'm guessing.
Clean all the surplus ink of the base of the print head and put it to one side. Now look at the platen (that's the long strip of metal that will be underneath the paper while the print head is passing). Clean that with a damp tissue. If you look carefully at that metal strip you should find a long wiggly piece of foam inset into a cut-out in the metal. Gently prise it out and wash it in warm water - take are not to stretch it or break it.
Now with a torch or lamp look at the right hand end where the print head goes when it is resting. That may well be messy too. Using dry paper towel try to absorb as much ink out of that area as you can (you'll get you fingers very dirty!).
When you think you have got as much out as you can use a damp tissue to clean off any dried on bits.
Now carefully refit the foam bit back into its slot - it will only go in one way - and then refit the print unit.
Close the lid and do a print. Hopefull y everything should be OK.