Blends are Color Transitions
A blend is the gradual transition from one color to another. Oil paint, because it takes time to dry, allows you to move the wet paint around on the canvas. This makes it easy to do the thing most difficult to do with other types of paint, the blend. All brushes will blend oil paint. Flat brushes are best and rounds the worst. The principles are the same for large and small blends.
The colors are mixed on the palette and applied in their approximate location on the canvas. The brush is then dragged back and forth in a crosshatch stroke between two values until a satisfactory transition is made. Parallel strokes are then used to refine the transition of values. A clean brush is used for the dark to middle and another clean brush for the light to middle.
(A) In a blend the brush strokes are ALWAYS perpendicular to the light. In a circular blend the brush must rotate to remain perpendicular to the light so curved brush strokes are used.
(B) The placement and size of value shapes within a blend create the contour of the surface. Note the value placements for flat surfaces on the left and curved surfaces on the right.