If all hakis cancel out and it's devil fruit vs devil fruit and blue fire is at 3000°C and magma at 1300°C, the blue fire is going to burn the magma.
Magma is just melted rocks so it sits around 1300°C.
If you burn magma at 3000°C, you are not only melting the rocks, you are also turning them into vapor, you are actually countering it.
I Googled the temperature for rock to turn into vapor: "It depends on the type of rock. Many silicates form a vapor at about 2800°C.
The blue lava you are talking about is an illusory-lava, it resembles lava but it's not lava so it's not magma.
Also, to have "truly-blue lava" (like stated in your message), you'd need to reach 6,000°C and this is impossible for rocks because rocks turn into vapor at this temperature so you are not having lava/magma anymore.
So yes, magma can't be blue.
Magma is just melted rocks so it sits around 1300°C.
If you burn magma at 3000°C, you are not only melting the rocks, you are also turning them into vapor, you are actually countering it.
I Googled the temperature for rock to turn into vapor: "It depends on the type of rock. Many silicates form a vapor at about 2800°C.
The blue lava you are talking about is an illusory-lava, it resembles lava but it's not lava so it's not magma.
Also, to have "truly-blue lava" (like stated in your message), you'd need to reach 6,000°C and this is impossible for rocks because rocks turn into vapor at this temperature so you are not having lava/magma anymore.
So yes, magma can't be blue.