Are nightshade blackout taillights legal?

No, It may not be so obstructed by a screen, bar, auxiliary equipment or device of any kind as to obscure, change color of or obstruct the beam.

Title 67 Chapter 175 Vehicle Equipment and Inspection Section 175.125. Lighting and electrical systems.

(e) Condition and position of lamps. Lamps shall be properly fastened; direct light properly; be of a color not contrary to regulations relating to required motor vehicle lighting equipment; location of required equipment; and required motor vehicle lighting equipment); and may not be so obstructed by a screen, bar, auxiliary equipment or device of any kind as to obscure, change color of or obstruct the beam.