Jeez, the more I read this passage, the more I get confused.
You asked:
"If craft unions exclude Black workers, then they would be less affected by right-to-work legislation than industrial unions, right?"
I think you mean "in a R2W state, black workers would be more impacted by lower wages for industrial jobs than for craft jobs". This is probably true.
However, in R2W states, craft jobs are potentially just as impacted by lower wages as are industrial jobs.
Lines 25-29 make it sound like wages, statewide, are impacted negatively in R2W states because the overall leverage of unions is weakened.
So the wages of craft unions AND of industrial unions are both negatively affected by R2W legislation.
If (D) said "in R2W states, the wages
of black workers are less negatively affected in craft unions than in industrial unions", then I think you'd be closer to having the inference you were trying to make:
Black workers must be LESS affected by craft unions, because they're typically not even IN craft unions.
But (D) refers to all workers.
The white workers in craft unions in R2W states are potentially just as affected as the white/black workers in industrial unions in R2W states, again, because of what lines 25-29 is saying.
Lines 32-40 are basically saying "unions are really good for black workers. They proportionately help black workers more than white workers. A black worker in a union vs. not in a union is a bigger disparity than a white worker in a union vs. not in a union. So if you want to find the negative impact of a R2W states, the easier place to look is minority wages."
The craft vs. industry thing is really just a sideshow; the author is saying we have to account for the data noise we'll get from the fact that black workers are typically only found in industrial unions.
Since (D) is about "all workers", it really has nothing to do with this distinction.
Hope this clarifies.