I don't know if Natsu would be able to consume Hellblaze. Hellblaze could burn down the Fairy Forest, which was immune to normal non magical flames, Hellblaze also can't be put out through conventional means, only very specific magic Merlin uses, and maybe some other magicians.
Edit: Holy Magic can also put the flames out due to breaking down Darkess particles, but i don't see Holy Magic on Natsu's page.
Meliodas' full counter works on much more powerful attacks. There's this guy called Ruin, Meliodas easily one shot him, Meliodas then broke into clones with the exact same power as Ruin and blocked several attacks from Albion, who is many times stronger than Meliodas' full power, and even stronger than his Demon form. Merlin said the attack uses minimal strength and power. Meliodas can also block non physical attacks, such as shockwaves,
Status effects,
Hellblaze, explosions, earth magic (even trees), fire balls, etc, and more than doubles its power, it's stated to be several at one point, so it could be higher. Full counter doesn't work against time based/delayed attacks however.
The Dark Cocoon feat of Meliodas was done when he was only barely transformed, and a heavily suppressed Meliodas far surpassed that amount of power, which doesn't even hold a candle to his full power, which was seen when he easily beat Escanor, who was comparable to his suppressed state.
I am not saying Meliodas wins, just giving details, i don't know enough about Natsu to give judgement.