Yup you are absolutely right two devil fruits which are adversaries of each other in many perspective, that is in it’s ability as well as the user’s of the fruit. Pika Pika no Mi eaten by Borsalino also one of the Navy admiral known as Admiral Kizaru and Yami Yami no Mi eaten by Marshall D. Teach who goes by Blackbeard Captain of BlackBeard Pirates.
Pika Pika no Mi allows its user to control, create and transform into light element as it stands for, on top of it its a Logia type devil fruit which are considered the strongest type of devil fruit, allowing its user to turn into the element of that fruit. Current user of the fruit is admiral Kizaru.
Yami Yami no Mi same goes for this devil fruit where the user can create and control the darkness, the current user is Marshell D. Teach a.k.a Blackbeard who has actually stolen this fruit by killing his former crew mate Thatch who was the 4th division commander of Whitebeard pirates.
You can’t only say that the fruit’s are of exactly opposite nature or ability but also the user of the fruit are opposite by their work, where one is a pirate and other being a Navy Admiral.
Admiral Kizaru is one of the three Marine admiral and the only one from former trio admirals. His character is cool and composed, his fighting skills are top notch as we have witnessed it in Sabaody and in Marineford war. He has bunch of techniques utilizing his devil fruit power like Yata no Kagami , Ama no Murakumo or Amaterasu and more. The top ability is the speed of light which cant be beaten by anybody in One Piece Universe, making him the fastest man alive( you could say flash of One Piece).
On the Other hand Blackbeard aka Marshall D. Teach is a pirate formally a part of Whitebeard pirates, after killing his own crewmate for devil fruit has formed his own pirate group known as blackbeard pirates. He was made Warlord after apprehending Fire Fist Ace , where he tricked marines to isolate empel down for the execution of Ace, so that he can recruit members from the prison. He has shown what of a deadly power he has of the Yami Yami no Mi by defeating Ace. Blackbeard is currently one of the Yanko as well as has another devil fruit power stolen from Whitebeard that is Gura Gura no Mi.
Now what if Kizaru and Blackbeard go against each other in a battle, who will win?
Both are hell of a strong one being Navy Admiral and other being a Yanko, if they go into a fight with each other then it would be a fight similar to that like between Akainu and Aokiji, here Light fighting Darkness.
For Kizaru he is a top class trained marine went under hardcore training specializes in fighting even if he didn’t had his Pika Pika no Mi fruit, but for Blackbeard he is made up of something else only cant say that he is well trained but may be good at fighting, but he is more of his devil fruit power.
The advantage for Blackbeard is that he has two devil fruit powers one is Yami Yami no Mi and other Gura Gura no Mi, the later being the disastrous one, literally capable of producing disasters.
But their battle might be equally matched leading to hand to hand haki battle, the stronger the haki might just have the upper hand, as we have seen that devil fruit abilities not always have advantage over haki as haki is used to counter the devil fruit abilities like Logia, here both being Logia type.
But if there were really any chances for the fight between Kizaru and Blackbeard the Navy might not approve it, as Blackbeard has literally become a strong force which is feared.
You might say that their powers are equally matched as both are experienced in fighting or using their devil fruit power to the full extents. But I would say the stronger haki user might just have a edge over other.
Thanks for reading
kizaru wont dare to go against bkb as he is one of the yanko