Why might you get terrible gas mileage and black smoke come from the tailpipe of a 1994 Chevy S10 with the W series engine?

Engine is running way too rich. Have this fixed. Damage to the O2 sensor and the Catalytic Converter will occur. Check for clogged air filter,leaking fuel injectors or clogged throtle body. Check the map sensor and also check the fuel pressure regulator for a leaky diaphragm. If there is fuel in the vacuum line it needs replacement pronto, then change your oil as it will be contaminated with gas.