Quote from Kaitentsuki
After Whitebeard died you may certainly be right, but during the time skip we have no idea how much Blackbeard has grown and how many powers he has attained.
I'm not a 100% positive on this, but I'm pretty sure Blackbeard was only looking to obtain only Whitebeard's power, in regards to the time skip, it's possible he became a Yonkou. But also I think it was stated to be impossible to have multiple devil fruit abilities, even if Blackbeard has 2 now, going beyond that could kill him. Especially since it almost did trying to get Whitebeard's.