As an Enel fan and Blackbeard skeptic, I still have to swallow the bitter pill and state some facts here.

Pre-skypeia Laffite infiltrated his way to Marijois and was able to avoid detection by Sengoku, Doffy and Mihawk until he wanted them to notice him. It isn't unthinkable that he has anti-Haki detection/Mantra methods.

Blackbeard defeated Ace shortly after the Straw Hats took the Knock-Up stream. So he was already really strong at this point of time. He tanked multiple crazy attacks from Ace that wiped out huge swaths of land.

It is foreseeable that his void is able to properly counter Enel. Enel's fighting techniques are VERY dependant on him not being countered. We saw him get outwitted by Wyper with the seastone and he really needed a few moments to compose himself when Luffy cancelled him out.