Abrush called a filbert is best used for

It is best used BLENDING.
filbert brush is a kind of paint brush commonly used in artworks.
It is a brush with a flat but oval head shape.
comparing to regular flat brushes, it is a lot more easier to blends colors.
it is even suitable for painting either thin or thick strokes.
Even though it is normally harder to control, but it gives fluent and smooth brush strokes.