Слова встретились в стандарте ASTM, определений нет. Примеры контекста:
Pipe produced in single length: if any test result fails to conform ..., the affected single length shall be rejected.
Pipe produced in multiple length: if any test result fails to conform ..., the affected multiple length shall be rejected, or tests shall be made using a test specimen taken from each end of each individual length in the failed multiple length.

Слова black pipe встречаются только в названии стандарта:
Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc-Coated, Welded and Seamless.