Feel blah, living day-to-day, lack zest for life, feel more "empty" and unmotivated than sad. could this be a symptom of depression?

Absolutely.: Some people manifest symptoms of depression in different ways. Some are tearful often then others who stop feeling happy and instead have this somewhat "numb" feeling. Sometimes this feeling precedes a more obvious kind of depression, but not always. I would make sure you talk with your doctor about this. There are many good treatments that could help you. Don't suffer in silence alone. Be well!