I know it’s an honest question, but this is very hard to answer as you put it.
As @lexar4h1 mentioned, makers have certainly stitched good outseams using blades that other makers use for inseams. The main thing is that it’s sharp.
An inseaming blade doesn’t know it’s an “inseaming awl” or a “stitching awl” or a “heel awl”. Some makers, like Osborne, don’t even market their blades with those labels. A blade is made from a particular steel, in a particular thickness, bent in a certain way, ground a certain way, &c. &c. &c. Some call particular combos of those characteristics “inseaming awl” or “sewing awl” or “outseaming awl” or “stitching awl”, based on their preferences and expectations.
Your best bet is probably to find someone who’s handstitched stitchdown boots and ask them what they used and liked, in terms of thickness, curve, grind, point, and sharpening. Dennis Kieback, for example. Then find that, or something close to it, for yourself.
I haven’t done stitchdown myself. But I wonder whether outseaming those is really all that different than outseaming welted. Especially if you’re not trying to stitch welted really close in, under the upper.
I’ve personally had best results with square awls—that’s a grind style—from Tom Carbone and Dick Anderson. Either of those are readily available mail-order in the USA. They’re both clearly inspired by older English and European blades. Japanese toolmakers also seem to follow those patterns for stitching awls. The ones I imported from Mamoru, for example—you can find photos of those here on the forum.
If square awls weren’t available, I would probably look for thicker diamond-point awl blades, like saddler’s awls, but curved. Especially for robust rather than dressy styles. Some diamond-shaped, curved awls get called “German” in various sources. That might be another search term for those.
If you’re looking for inspiration, there are tons of great videos with hand sole stitching on YouTube. Including some doing boots. Dennis I mentioned. Bad Hat Brothers and Songlim in Korea.
I didn’t mark my notes of videos as having hand stitching from the start, because the beginning I was focused exclusively on handwork. But I’ve gone back and updated a few entries to flesh out the list here: Hand Outseaming | shoemaking.wiki