You know what really screws this up? Darkvision. If someone with Darkvision is in the space, and because you made the illusion "black(darkness)" they would not be able to see though it. That would let them know it's not really "Darkness". Now if you mean you make it seem like the Darkness Spell from that elemental, anyone who is not in a 3x3 cube (15foot cube) would also know, if they know what the Darkness spell is, that something is not right. The area would be in a cube shape and not like a cloud. The area is really small too so anyone could get out of the area if they move 15 feet in any direction. But hey, that is what came to me when I read this.
. Spells that require concentration should often give more bang for their buck, and having not known about booming blade (don’t know what book that’s in, but couldn’t find it in xanthar or phb),
You know what really screws this up? Darkvision. If someone with Darkvision is in the space, and because you made the illusion "black(darkness)" they would not be able to see though it. That would let them know it's not really "Darkness". Now if you mean you make it seem like the Darkness Spell from that elemental, anyone who is not in a 3x3 cube (15foot cube) would also know, if they know what the Darkness spell is, that something is not right. The area would be in a cube shape and not like a cloud. The area is really small too so anyone could get out of the area if they move 15 feet in any direction. But hey, that is what came to me when I read this.
You can find my published homebrew Spells here.
Just FYI, a 15 foot cube is a cube 15 feet on a side; 15 cubic feet is a volume that could be a cube, and if so, would be about 2.5 feet on a side.
Green flame blade and Booming Blade are both from the Sword Coast Adventuring Guide or SCAG.