Our laundry used to drain and then back up into the shower stall in the next room. We have since removed the shower stall to re-use the space, and have ripped down some walls. The pipes are now exposed. The laundry drain pipe seems to empty into the black vent pipe, and not a metal main drain pipe. There's also a terrible smell whenever the washing machine empties - wondering if it's something we can fix. Please teach me... Heather :)

The laundry drain pipe seems to empty into the black vent pipe,
I have put up a rough drawing of a washer drain,trap and standpipe,(see image). Notice that it looks like it's draining into the vent but actually goes into the drain line.

There's also a terrible smell whenever the washing machine empties
This is caused by back pressure in the drain. The back pressure can be a partial clog or simply a too forceful pump and a too small drain line. One way to eliminate to smell and be certain that there will be no backups out oif the stand pipe would be to install a compression fitting,(see image) on the stand pipe and contain both the smell and the water. Good luck, Tom