Considering that the Fire Dragon God and the Fire Spirit King designed the Trial by Fire to raise one's fire affinity, Vaan didn't doubt the Fire Dragon God's blood essence had such an effect.
The blood essence of a Rank 6 being should be full of wonders.
Of course, if he had a high fire affinity talent, the best way to maximize the Fire Dragon God's blood essence would be to temper his body in magma until his natural fire affinity couldn't be raised any further.
If he took the Fire Dragon God's blood essence at that point, it would likely boost his fire affinity beyond the bottleneck.
However, the reality is that his body had no talent for fire affinity since he had acquired the Earth Spirit Body. The Earth Spirit Body made it more difficult for him to raise his fire affinity.
That's why he had to absorb the Fire Dragon God's blood essence early to develop his fire affinity talent.