I have a car that has been sitting for awhile and the oil is black, so I thinking of adding one of those engine flush additives
(not that seafoam stuff) when I change the oil. My buddy mentioned that it could cause the seals to leak. Any truth to that?

kind of a stupid question but I thought Id ask anyway.

depends on how many miles are on the engine. but just running some engine cleaner through it shouldnt really cause any problems. the only problem i have ever had with this kinda situation is flushing the cooling system causeing a head gasket leak.

Cleaners don't cause leaks - they expose them. Sometimes a neglected engine can have enough sludge built up to create "false seals" behind the real ones, which prevents oil from getting to the actual seals and wears them out quickly. When you run a cleaner through and it softens or removes this sludge, then the oil can get to the seals which are now worn out, and that's when you find your new leaks.
I would probably just give it an oil change, and then perhaps another after 500-1000 miles or so, depending on how quickly the new oil darkens. Oils have detergents in them already, otherwise we would never get the mileage out of our engines that we do now. 50+ years ago this was different, and valve jobs/rebuilds every 30,000-50,000 miles were considered normal.
I would still use a "name brand" oil for this, but obviously don't bother spending money on anything too premium as you're essentially just using it as a flush. Make sure you change the filter each time as well, as it may plug quickly if there's a lot of sludge getting dislodged.