Blending is the process of mixing two or more colors to create a unique and realistic color on paper. It is used to add details to the subject i.e highlights, shadows, base color, color gradients, etc.
A key rule for perfect blending is applying layers after layers on top of each other. Always remember, patience is the key. Start with a lighter color and apply a layer with very little pressure, leaving gaps in paper. Repeat the process with darker shades till all the gaps are filled and you get a clean blend. You can then use a lighter shade on top of the darker shades to create highlights or smoothen the shade.
I personally use prismacolors and polychromos for my drawings. Polychromos are oil based and Prismacolors are wax based. Blending with prismacolors is much easier. Polychromos on the other hand, have hard tips and are good for detailing and outlining. Also, the paper you use with color pencils matters and should be optimal for color pencil drawings. Always check the paper before buying.