I think it's just a bug (possibly a lightning beetle?) and that it has nothing to do with the spots you get on the mint leaves which are probably fungal or bacterial speck of somekind and which, in any event, are completely unlikely to slow down the doggone mint. My mint gets a little tatty looking much like that as the summer wears on. I suspect the heat and relative drought stress the plant and make it more susceptible to the foliage disease. Whatever. Again, it isn't doing the mint any real harm in the long or short run. Were I you, I wouldn't worry about the bug (which would probably be perfectly capable of making its way to the flower bed even without the proximity to the mint). Instead, I would worry about installing some kind of root barrier to keep the mint from invading the flower bed -- or at least slow it down.

Maybe this guy is not the culprit, but only looks guilty.
Kiki

I'm guessing it's spittle bugs - little orangish red bugs that leave dark brownish/black spots on the leaves. Hateful little critters that turn beautiful mint into a spotty mess. They never seem to bother much else - just mints and balms.

Thanks, I'll check it out,
Kiki

I wish something would eat my mint.

Sometimes plants that spread a lot get very crowded in the roots over time and it causes them to stress out because they're not getting enough nutrients, water, etc as they need it, so they get spots and turn black etc. I had some "Snow on the Mountain" that got very black and ratty looking every August because it's in a dry spot and very crowded. I tninned it out.. actually, I pulled all of it out, but it's still there.. just a lot healthier looking now. I bet if you dug up the mint and pulled nearly all the roots out, it might be a lot healthier looking. As to keeping it out of your flower bed... good luck!

If anyone's reading this four years later (because you have a similar problem) I suggest you Google "FOURLINED PLANT BUG".