1995 Toyota Corolla black smoke

I have a toyata corolla 2e engine that was recently overhauled. Piston rings were replaced with standard size since the piston space were still tight. Replaced parts were piston ring, valve seal, cylinder head gasket, sparkplugs, oil seal, oil filter, oil. After overhauling I've noticed that there were still oil leak under the oil pan but the engine performance that time was good no smoke, good acceleration, good timing. I've returned the car for back job. They've fixed the oil leak, but I've noticed the performance went bad and black smoke came out, the engine sound slight scratchy, not fine. So I've returned again the car, but still there's a black smoke coming out.