Blending Medium is used to slow the drying rate of watercolours allowing you more time for blending.
It is particularly useful in hot climates where artists would like their water colours to stay open and workable for longer periods of time.
Blending Medium can be used in a number of different ways. For maximum blending time, mix the medium directly with the water colour. Alternatively you can apply the medium directly to the paper in preparation for the watercolour wash. Dilution with water will provide a variety of blending/drying times. Drying rates will vary and depend on the temperature and air flow. Once dry, further washes can be applied over any washes which included Blending Medium.