I applaud your thoughtful research into using a shade tolerant rose like Zephrine and I approve of adding the clematis to the arch. I did have an issue with black spot on this thornless rose when it was young, but I solved the problem by removing any leaves closer than three feet from the ground. This means my rose has "naked knees" but the clematis fills in at the base of the plant. I also use a composted dairy manure mulch each spring at the base of the plant and this has made a big difference as a fresh mulch will seal in any overwintering black spot spores that rest in the soil. The lack of lower leaves also keeps the upper foliage clean as rain cannot splash the spores onto lower leaves where it can travel up the plant.  In answer to the third question, yes I like the combo of Zephrine and 'Multi-Blue' and think you will have great success with blooms on your arch if you are patient - young plants are just more susceptible to disease.  Thanks for listening to the webinar and Keep Growing, Marianne Binetti