STRANG3WB

Alright how's it going everybody?
I've got a quick question and hopefully I can get a quick answer from someone. I have a '95 k2500 Silverado with 257,xxx some odd miles on it, runs very strong and is very sound. Lately I've been noticing on a cold start up black soot coming out of my exhaust. Only reason I'm noticing this is because I have a remote start for my truck, anyways I've been going through months and months of trying to diagnose why it's doing this. I've replaced my throttle body gasket, plugs, plug wires, cap & rotor, egr valve and have done routine maintenance on it such as oil changes every 3,000 miles. I've also ran seafoam through my gas tank when I filled up to hopefully clean out whatever it may be that's causing this black soot to shoot out of my exhaust on a cold start up. I've heard it could be anything that's causing it, whether it be my valve stem seals or bad rings in general. Now I'm thinking that it could be burning excess gas that's left behind considering that the truck does have 257,xxx miles on it. Any ideas on what could be causing this and if it is carbon build up how should I clean it out etc...What cleaners are best to use on such a thing? I've heard nothing but good things about seafoam cleaning up any gunk that may be in the motor but I've only used it in my crankcase and gas tank on previous vehicles. Would seafoam be good to run through the brake booster vacuum line to clean this carbon build up crap out?
Thank you for reading this novel I just wrote. Any help and advice would be greatly appreciated