Any variety of soybean can be used green as edamame, as soon as the pods have filled out; but not all are suitable for that purpose. Some are too small to be practical, and others have clinging membranes or an "off" flavor. There are black-seeded edamame varieties (such as Black Pearl), so the seed color alone is not an issue.

I grow a lot of different soybeans, some of which are edamame, and others that are intended for processing. As part of my testing, I taste any large-seeded cultivars as edamame. Although some of the flavors & textures have been interesting, not many of those bred for processing have proven to be especially good. You can increase the chances if you pick the pods as soon as they have fattened up, but before any of the pods have begun to change color... the seeds will be at their sweetest at that point.

All of the soybeans planted at the same time, will reach edamame stage at the same time, and that stage may only last for a few days - so be prepared to either use them or freeze them at that point.