(hypothetical scenario) What if BlackBeard had stolen Marco's Mythical Phoenix Zoan, instead of Whitebeard's Gura Gura no mi?

Yami Yami no mi is a Logia with 2 main weaknesses.

1. You can't turn intangible like other Logias. This is hardly a weakness since the user was intangible anyway before obtaining the fruit, so it doesn't make you any weaker or more vulnerable.
2. It sucks in atttacks and causes the user to take much more damage and pain than normal. This is the main weaknesses of Yami Yami

Given how BB could tank attacks from Whitebeard, Sengoku, Garp even though he was taking more damage due to Yami Yami no mi, it is safe to assume that his raw strength, stamina and endurance is in the top class.

If he got his hands on Marco's healing, he would be pretty much impossible to damage, as he could tank it and heal back right up!