Can you say whats the difference between 4th grade Sandai and 2nd grade Wado?
Enma being turned to black blade changes nothing except its degree of hardness. Unless the other blades start breaking it is not a power-up of any kind.
That question should be directed to Oda not me. He's the one who is skipping those details we crave to know.
But judging by the fact that the swords are graded and have ranks. It would only be an ignorant act (with all respect) to ignore their ranks and say they are all equal, cause they are not. their ranks do matter, otherwise, Oda wouldn't have created those ranks, but again, the question should be directed to Oda, who instead of showing us details about Wado ichimonji's special powers or Sandai Keitetsu which was said to be a cursed sword, he chooses to show us the kids in Punk Hazard, and Tontatta and waste chapters and panels on characters we couldn't care less. While ignoring the explaination in details about one of the main character weapons.
Hopefully, in this arc, because Heitetsu said that Wado is also made by the same guy who made Enma, Oda can focus on those two blades and highlight a special power each one of those swords posses, we certainly witnessed now Enma having some sort of special power which drains Zoro's haki but when released in an attack, the power is maginfied, because Zoro intended to cut a tree, but ended up cutting the coast itself. So, there is maginfying in power at the cost of absorbing Zoro's haki and draining it, kinda like Gear 4th with Luffy.
As to Wado, I truly hope Oda shed more light to it, specially since it's made by the same guy, Shimotsuki Kouzaboro.