"The Effects of Ultraviolet Radiation on PVC Pipe"
Modulus of Elasticity = unchanged after two years of sun Tensile Strength = unchanged after two years of sun Impact Strength = reduced drastically (by 75%) after two years of sun Structural Integrity = unchanged after two years of sun Pressure Capacity unchanged after two years of sun
The loss of impact strength stopped deteriorating as soon as the pipes were protected by the sun so it's directly related to the UV radiation.
Luckily, the presence of an opaque surface between the sun and the pipe prevents UV degradation, since UV radiation will not penetrate thin shields such as paint coatings or wrappings or burial.
Why black? What's more opaque than black? Electrical PVC outdoor pipe is gray. Go figure.
PS: I heard some paint won't stick to PVC so maybe you need special PVC paint?