It is not easy. You have to open the flower and hand transfer pollen from one flower to the other. Technically to get a pure hybrid you have have to remove the pollen from the flower you wish to pollinate (to keep it from self pollinating) (That why there are no hybrid beans for production)

I am a firm believer that all named beans are either stable hybrids or mutations
There are just too many varieties to think otherwise
Very few resemble their wild ancesters
The catalogs are filled with new varieties each year.
They had to have been crossed by someone or insect.producing a hybrid
The word hybrid does not always mean crossing different species
Charlie

A stable hybrid is an OP. Lots of professionals do hybridize beans but only for the purpose of creating new varieties. It is very labor intensive. Once a hybrid is created then it is grown out for multiple generations until it is stable. Just not feasible at this point to make hybrids (and we are talking F1) in mass. Fortunately if you can create a couple of lbs of seeds, then you can commence the grow out and selection process. 7-9 generations to get an OP.

I would like to see a black seeded flat roma type bean as black seeded varieties do better in this climate than the regular romas,which we love
Maybe it is something some of us can work on.
Charlie

I doubt that the color of the seed has anything to do withit. But Black Seeded Marconi (Romano type) and Super Marconi (Roma type) may be a place to start your trials.

Super Marconi did well last year and is a complete bust this year besides we do not care for the taste as we do the white seeded
If I wanted to hybridize beans I would cross the black seeded varieties that was bred for the gulf coast states with the white seeded varieties bred in Holland, Germany and cooler areas
Charlie