What is the best fertilizer for blush nandina?

That depends on the results of a soil test. The nandina in question has been fertilized over a long time with Miracle Grow and the plant is starting to drop leaves. Using Miracle Grow repeatedly can result in the build up of potassium in the soil which can become harmful to the plant. A soil test can tell you how much is in the soil and give recommendations for bringing it into good ranges.

Related Videos:
Soil Testing
Fertilizer Basics

Related Resources
Nandina Fact Sheet
Nandina, Heavenly Bamboo (Nandina domestica)
Corrective Measures and Management of Over-Fertilized Soils