I have bought a built under oven,the colors are black and blue on the appliance.?

yes

Can you provide a model name or part number? Usually the colour coding are explained on the back of the unit or in the manual. Advise to do not start work and power up the oven without knowing exactly which is what. The blue, on 2 phase domestic installations represent neutral, black sometimes represent a so called "switch wire" and needs to be connected on the live side of the circuit. The safest bet would be to find out the manufacturer manual or open the connection unit between the cable mentioned above and the unit, check the label/markings (each conductor should have a specific acronym like E = earth, N = neutral, L = live or others). Please provide a picture of the cable and of the markings.