Leaving a short-term teaching position early
You need to talk with someone in the department about how to handle this (presumably the teaching coordinator, or chair, or whoever hired you).  In the sorts of departments/universities I'm familiar with, it would be difficult to arrange for someone else to take over a course partway through the semester.  Then your options might be teaching the whole course or leaving a semester early, and in the latter case they might need to know soon to line up a replacement.  On the other hand, it's possible that something very different could work out, depending on the local situation.


  But if I did that, would I piss the department off so much as to find myself in some way blacklisted in years to come?


Maybe, if you announce a decision without consulting with them and if that decision is extremely inconvenient.  However, it sounds like a reasonable thing to request, if you're flexible about trying to meet the department's needs as well (for example by quitting in between semesters rather than partway through the spring).