Too true. And often those packets contain inferior ingredients and/or 'enhancers' and/or stuff better added separately (salt, sugar) where they are both better able to be monitored and cheaper if simply pulled from your cabinet rather than included in a blend.
With the exception of complicated complex blends like raz el hanout with its two dozen ingredients (several of which needing prepping before grinding), berbere, achiote pastes and the like, I don't find the need to make blends ahead of time. However, if a chili blend is in the cards I'd suggest basing it on a chile with good depth, like guajillo, and building from there, perhaps with cascabel for it's berry notes and/or ancho for its lighter tones. Granulated garlic and onion are good additions but, frankly, I'd skip the cumin common to chili powder blends. I love cumin but its addition to the pot is best made toward the end of cooking (it could be included in the blend for the last 'dump'). Also, though oregano is common it is important not to use Mexican oregano because it cannot handle long cooking; use Greek or one of the other European oreganos. Better still, to me--forgo the oregano and use marjoram and thyme instead. Top-of-the-head suggestion: 3 parts ground guajillo, 1 part each ground cascabel an ancho, 1/2 part each onion and garlic and thyme, 1/4 part marjoram. Just a thought.