I've thought about this issue for quite a while and I have a hypothesis about both Shanks and Blackbeard.
Oda has portrayed Shanks as a reliable and trustworthy character (by saving Luffy the main protagonist at the cost of his arm, and by going out of his way and warning Whitebeard of Blackbeard's plan). Now I mention this because I absolutely believe everything Shanks stated in his monologue to Whitebeard is all 100% true.
Lets analyze what was posted by Zehahaha
Shanks: Whitebeard...I've gone through many battles and suffered various wounds, but what aches now is this one! This isn't a wound from adventures, nor did I get this from Mihawk; Your man "Blackbeard" Teach was the one who inflicted this wound! And it's not like I was careless.
Do you understand what I'm trying to say here Whitebeard!? He's been waiting for a chance...he didn't take a position of a captain or make himself known; all this time, he's been hiding in the shadow of the famous "Whitebeard!" - and now he's gained "power" and started to take action.
We learned during the Marineford arc that Blackbeard could have been the second division commander of WB's crew, but turned down the opportunity. We also know that Sengoku the fleet admiral of the marines, had never heard of Blackbeard when Laffitte nominated him for the position of Shichibukai. Shanks is accurate in stating that he's been hiding in the shadow of Whitebeard, and with his new "power" (referring to his devil fruit which we saw in action a chapter later) he was ready to take action (which he did by capturing Ace and invading Impel Down). That second part of Shank's monologue is completely true! Blackbeard in the end did go after Whitebeard's position and has in fact conquered for the most part a lot of his old territory.
Now if we go back and look at the first part of Shank's monologue we have no reason to believe he is lying about anything there as well. Shank's wasn't caught off guard by Blackbeard and his scar does in fact hurt him. While we don't know when exactly Shank's received his scar (Oda is doing his best to hide this from us) I believe the reason it is being kept secret is because there is more to the Shanks and Blackbeard relationship (i'll explain what I mean by this later in my post).
For now I believe Blackbeard has always been strong. We know that Blackbeard is aware of Haki. Specifically, we know he has observation haki. For example, he's able to sense Luffy's strength in Mock town and is perplexed by his simple bounty of 30 million beli at the time. He also knew a confrontation between Bellamy and Luffy would occur inside the bar. It would not be a stretch to say that Blackbeard is aware of the other forms of Haki as well, seeing as he was on WB's ship for more than a decade, but the question is why don't we see him use CoA or conqueror's?
My hypothesis is that Blackbeard is so in love with his Yami Yami no Mi powers that in the first half of the series he decided to only use his devil fruit abilities and not use haki at all. Blackbeard had finally obtained his beloved devil fruit after so many years of searching for it, that all he would do is just use his is Yami Yami no mi powers and avoid using haki all together (perhaps to experiment with the positives and negatives of his devil fruit).
Going back to Shank's I believe there is more to his relationship with Blackbeard. Shank's is known as the captain of the Red Hair Pirates and is known as Red Haired Shank's. Yet, Shank's jolly roger has no emphasis on his red hair, but on the scars given to him by Blackbeard. Interestingly enough the Jolly Roger Shank's uses has been around since the beginning when he was still wearing the strawhat (one would think his Jolly roger would include the strawhat since it is an important heirloom). I think the reason Shank's scar hurts him is because he might have respected Blackbeard a lot at one point (perhaps even influenced him to continue being a pirate after Roger died) and felt betrayed with what Blackbeard eventually did (killing a fellow nakama).
