Assuming this is BBA...
 

It started a week or two ago as tiny barely-visible black spots, and I increased my CO2 at the same time (my drop-checker registers yellow), but it is still growing.

What else can I try to get this to stop growing?

https://dl.dropbox.com/u/24247931/Aq...8/IMG_4943.JPG

Definitely BBA. You already cranked the CO2, but you can also try turning off your filter and spot treating with Excel. Check the forum, tons of solutions to try.

You can try trimming the leaves that are infected.

look like a slow growing plant :) BBA will most likely attack slow growing plant.
cracking up the co2 can cause BBA as well if the tank is light tech.
a fast change in co2, that leaf is cover in BBA. I say trim it and dose excel.