How do I increase the air flow to the burner to get a more efficient blue flame?

Are your vents, both outside and inside clear? Marco direct vent fireplaces has gone out of business and taken over by Lennox. You should contact a dealer near you for information. According to Lennox, any vent adjustments should be done by them, so very little information is available.

No owners manual found. Sorry.

If it is a direct vent, make sure the outside inlet is clear. I have seen mud dauber and paper wasp nests in the housing.