Dennis- I'd place a hard card between the plastic cup and the powder, if I was you. Black powder flame is considerably hotter, it seems than smokeless powder.
Smokeless powder fired plastic wads will show marking, but not the melted plastic that black powder seems to produce.
When using plastic shot cups in 12 bore BP loads, I found the black powder actually melted the plastic cups and coated the bore with plastic.- thus my recommendation of using a card to protect the 'cup' from the flame.