I started a new character on Ridiculous to test this and captured some video, but the file size limitations here make it impossible to post. Is there somewhere I can send it?
The result is the same. At level 2 of Radiant Blast, initial skill damage is done. At level 3, DoT damage is applied, but initial skill damage is only done to enemies that alread have the DoT, making it harder to kill enemies once leveled.
I would suggest you test this on Ridiculous as easier difficulties will have the enemies dying to the DoT instantly, making it seem like they were hit by the initial blast. With high crit and damage in the first area, I had this happening to some enemies even on Ridiculous.