I don't think using the taper command on your model as is will get what you want. There are, however, numerous ways to get what you want. The bottom line is, you are going to need 2 faces so you can taper the one you want and leave the other one alone.
3 ways (that I can think of off hand) to get 2 faces: 1) build the block in 2 phases, build the part you want to taper, taper it, then build the other half and unite it together 2) use the split face command to split your existing face into 2 parts and taper the half you want 3) build a trim tool in the desired position and trim your existing solid.
Which should you use? that depends on what other features this part requires and your modeling preference. If tapering the face is the last thing to do, splitting the face is probably the easiest way to go. I prefer not to split a face if there are a lot of features to add that will/may become child features of the split face. This just makes editing easier if the split face is no longer needed in a later revision (which I have seen happen often enough). In this case I would probably build some sort of trim tool.
