How does it compare to Sanji's blue flames?
It's really hard to say.
You'd have to estimate how much more durable Sanji got thanks to the exoskeleton, and since this new durability is what allowed him to withstand much hotter flames, then the multiplier for his heat should be about the same as for his durability.
Starting with the regular Diable Jambe, I personally think there's a good argument to be made that Sanji's Diable Jambe was as hot as the Mera Mera no Mi's flames.
There's also the fact that Ace had a very hard time overcoming Jinbei and Whitebeard's haki with his flames despite Whitebeard being sicker and sicker.
At first Jinbei even thought it was impossible for Ace's flame to burn through his haki.
Even in Marineford neither Ace nor Marco's flames seemed to be hot enough to go through any of the Admiral's advanced haki (even though Marco can burn Prometheus).
On the other hand Kaido's Boro Breath could indeed go through Luffy's advanced haki, even once Luffy unlocked Conqueror's (although Luffy could withstand the heat).
So basically Kaido's regular Boro Breath seems much hotter than the flames from the Tori Tori no Mi and the Mera Mera no Mi.
So Kaido's regular Boro Breath should be clearly above Sanji's Diable Jambe.
But with the Ifrit Jambe Sanji is on a completely different level.
I think the heat of his blue flames should be between a regular Boro Breath and Kaido's new Boro Breath in his Fire Form.
That's because Kaido's new form actually looks more like what King could do which, as Zoro pointed out, seemed to be at a state above that of flames.
So even though Sanji has special flames probably hotter than any other flames, as he says himself, Kaido has reached a higher state which is described by Zoro to be like magma, but I personally think it's actually plasma.
But I have to say that I don't know what's the gap in temperature between Sanji's Ifrit Jambe and Kaido's new Boro Breath.
It could be slim just like it could be enormous, it's on you to interpret things
So in my opinion :
Flaming Kaido
<
Ifrit Jambe
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Marco's Flames
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Ace's Flames / Diable Jambe / Red Hawk