Hi Ruth,
I usually use fan sable brushes to blend. I have also use synthetic nylon brushes and they do a gret job too. A couple things to remember though when blending oil:
1. The paint being blended has to be lean enough. Too much oil or medium will cause the paint pigments to run around.
2. Make sure the brush used for blending is dry or has minimal oil, medium or turps in it.
3. Clean the blending brush between blends.
I hope this will help you.
Good luck.