Blackbeard spent 40 years searching for one DF, he didn't want to settle down for any other DF other than "
STRONGEST LOGIA." Then he came up with a plan to start a war that would weaken Whitebeard, he broke the most secure prison [impel down] endangered his life and almost died from Magellan's hands, went to to Marinford and fought against Whitebeard, Sengoku and Garp. And all of that just to eat "
THE STRONGEST PARAMECIA" in the world!!! He could have eaten any other strong DF that would be easier to get, but no,
Blackbeard is hunting for THE STRONGEST DFs IN EACH CATEGORY: STRONGEST LOGIA, STRONGEST PARAMECIA, STRONGEST ZOAN. The only way Blackbeard doesn't take Kaido's DF, is if Kaido's DF isn't the strongest zoan,
which I highly, highly, highly, HIGHLY doubt. The only character who can have the strongest zoan is Kaido.
Plus, Kaido's DF is perfect for eliminating one weakens that Blackbeard has:
his lack of durability. Blackbeard lacks durability, Kaido's DF has the durability that Blackbeard needs. On top of that, Blackbeard's Darkness Darkness can weaken DF users, his Gura Gura has the highest destructive power, but Blackbeard lacks offensive power, and Kaido's DF offers exactly that.
Luffy is familiar with every DF Blackbeard has, he has experienced Whitebeard's DF in Marinford
He has experienced Blackbeard's DF in impel down
There's no secrecy about Blackbeard's ability, Luffy knows what powers Blackbeard has. Luffy will not face Blackbeard blindfoldedly, the exact argument you use against [Blackbeard eating Kaido's DF] is the exact reason why he will eat it.