I'm troubled by all the air bubble pockets (the little white circles) in a blank you purchased. That's a casting flaw. An occasional bubble sometimes happens, but anybody who is selling blanks should be notified of that many, and you should receive a replacement or a refund.
What I'm seeing might not be an issue with your preparation. That grayish base color for epoxy should be opaque enough in this instance that the tube should not show through. I speculate this is the result of the maker not mixing the base color well enough (used white and black to make gray), and the white colorant is the culprit.
I don't cast for dough, and prefer to not criticize other casters. I'd be embarrassed if a blank like that left my shop, and I'd certainly want to know if it did so I could make it right.