All of the fittings on the Blichmann conicals seal with o-rings. The 90 pipe-elbow seals with the fitting with a tapered face. The fitting then seals with the outside of the conical with the o-ring. No tape or sealant should be used. If a leak is occurring, it is because the o-ring is not sealing with the outside surface of the conical. This is the same on the bottom dump fitting and on the racking valve.
One thing to check before filling the conical is to make sure the o-ring is fully seated against the outside surface of the conical. If the fitting isn't tight enough, the o-ring will not seat, and leaks may occur.