The description for the green-flame blade spell (SCAG, p. 143) says:
As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range.
It doesn't specify that the "attack with a weapon" can't be a "spell attack with a weapon".
Does the effect conjured by flame blade count as a weapon or not and therefore be used in conjunction with green-flame blade?
It doesn't say in the spell description of flame blade. It only states it is similar in size and shape to a scimitar which sounds strikingly like a weapon to me.