Black smoke at WOT (wide open throttle) means that your car is running to rich. This is due to either too much fuel pressure, flooring the throttle at low RPMs (this is how the computer compensates so the engine doesn't ping or detonate), or just excess carbon being blown out of the exhaust that accumulates during normal driving. Fresh plugs aren't going to fix a rich running engine, but as far as recommendations go, the NGKs or Nippondensos are the plug of choice. I like the bosch platinum 4419s myself in my 93 si, but I had bad luck with regular or dual platinum bosch's. I recommend checking your fuel pressure (should be between 35 to 40 psi at idle with vacuum line attached to regulator on a 92-95 D16Z6). I assume your car is a stick and not an auto. Usually you will get bad gas mileage with an overly rich running engine. A lot of aftermarket computer chips will make the engine run way too rich... they are typically made for heavily modified engines. I've seen it more common to see black smoke from cars without a cat, since converters tend to light of any extra unburned fuel that comes from the engine. Under normal driving after about 500 or so miles with correct plugs in your car you will see a light tan to slight grey discoloration on the ceramic part of the plugs. If the're black and sooty, there's a problem. Also check your ignition timing. If I'm not mistaken, you should be running premium gas in your EX (D16Z6 ?). Let me know if you need more info.