Actually, a lot of them do list easy swaps in the recipe notes. I always look down there before I head to the store. It saves me from buying a whole jar of something I'll never use again.
Last Tuesday, I needed cumin but only had chili powder. Checked the notes on this soup blog, and they said it was fine to swap. I do that all the time now, saves me so many trips.
Man, I gotta push back a little here. Tons of recipe bloggers actually do list substitutions in the notes section below the main recipe, but a lot of people just skip right over it. I've saved a fortune by checking there first for spice swaps or using lemon juice instead of some weird vinegar.