If the Everlasting Blood Mantra was about drawing out the latent power hidden within the blood and accumulating it to create stronger blood power, then the Divine Blood Refining Art was about extracting existing strong blood power and absorbing it.
Although the blood fusion aspect was also covered in the Everlasting Blood Mantra, its effects were limited.
Thus, even though the Divine Blood Refining Art was far inferior to the Everlasting Blood Mantra, it still had its strength due to the difference in their specialties; it had strong blood fusion effects, albeit with some limitations of its own.
The Everlasting Blood Mantra focused on accumulation, which took time and was limited by the readily available and accessible origin blood.
On the other hand, the Divine Blood Refining Art focused on sheer power growth. As long as there was high-quality blood power to extract and absorb, the blood art would grow rapidly.