Could this be the M.A.C #226 big sister?
Brush Set: $20
226 Small Tapered Blending Brush
What it is:
This brush applies and blends shadow within the creases to add depth.
What it does:
Known as the "windshield wiper" brush, this style has a tapered tip that softly blends back and forth within the creases. This plush brush features the highest
grade bristles possible, for a supple, smooth application.
Price: $24 (image from sephora.com)