Modern Vespa

The PO of my bike used BR7HS plugs, and commented the plugs frequently need to be replaced.

The reference manual I got called out BR8HS plugs so I figured I'd try them, but they do turn black fast so if considering going back to the 7s to hopefully keep the plugs from fouling so quickly.

What other effects/consequences will result from that in terms of starting, idling, etc?

It's probably not your plug, but your jetting.. too rich! Especially if he was fouling #7 plugs often.

So I'm running to rich, therefore I need smaller jets?

Correct. When was the last time it had a proper carb cleaning and rebuild? If the answer is "I have no idea", I would do it while you're already in there.

Two weeks ago. I guess I don't know how proper it was but I followed this video...

new gaskets, bloat, needle, etc.

It runs and idles fine, the only issues are that it starts a bit hard for my diminutive wife to start easily, and the plugs seem to turn blackish rather quickly.

personally I don't worry about the plugs turning black as long as they don't foul.

Not yet.

Understand that the BR7HS plug is one step hotter in the heat range of that specific plug type.

Right. I'm hoping that will make it easier to start and keep the plugs cleaner.

Again, D.. don't go chasing a fouling problem with "hotter" and "hotter" plugs... If your bike calls for an 8 plug, you simoly need to get the jetting right so you're not fouling... as you said the PO was fouling 7 plugs so it sounds like your jetting is really rich... get your jetting sorted and don't try and plug around it! know what I mean?

I intend to look into the jetting for sure. I suspect it's more involved than just swapping out one jet for another.

When people speak of 'jetting' do they mean one of the brass jets inside the carb or dialing down the mixture screw.

I've taken apart my carb and recorded the numbers of the parts inside. I can try to find a dellorto dealer in the area or order parts.

It depends on the type of riding you do too. Short trips around town will blacken your plug quicker. Longer rides with the throttle open will burn the shit off. The colder plug might be alright for the latter.

"riding around town" will not be affected by either a B8 or B7 or B6 plug. That rating system NGK has is a measure of temperature response, the ability of the plug to shed/maintain a heat profile.

Al Gore created the internet just so he could ask about "hotter" and "colder" plugs.

Also, the OP needs to clarify what he means about plugs fouling. Does he mean the scooter gets hard to start and begins quitting as a result? Or is he just pulling the spark plug and thinking, "My spark plug is so dirty! It needs to be changed!"

For NGK, the lower the number the hotter the plug.
If you do lots of short distance, then consider hotter.