OK... got no replies... but still needed to solve the problem.
After much experimentation, what worked best was a small bucket of mineral spirits and a soft plastic brush on a stick. Use a small amount of minerial spirits in the bucket because it will get dirty fast.
Dip the brush in the mineral spirits and scrub along all the seams from top down, and do it a section at a time because you will need to rinse it off with the garden hose. Repeat if necessary. While it's not as pristine gray as freshly applied... it's no longer black.
Just remember to change the mineral spirits when it gets too dirty or be prepared to scrub smaller areas and rinse them with water more frequently.
Also... the mineral spirits will probably dissolve the brush over time so be prepared to toss the brush when you are done. My brush had about 3" long bristles when I started... and about 2" long when I was done.