Great Britain 1p Red #3 Or #3f Blue Paper? What Is This Blue Frame On The Back?

Hello all Stamp Masters,

I got this here Great Britain 1 Penny Red, #3 I assume.
Yet the paper is in bluish color, not sure if that qualifies it to be the deep blue paper of #3f?

Also:
When looking at the back there seems to be a blue "frame" around the design on the front.
What is up with that? I have never seen that before...

To prevent removal of cancellations. Many stamps were printed in double-fugitive (water and chemical solvents) inks. For the Penny Red, this was done by adding prussiate of potash to the ink. This reacted with the water in the dampened paper and caused the paper to become blueté. The Blueté paper stamps have completely bluish paper. Your example seems like a chemical caused the prussiate of potash component in the ink run and create a blueté shadow. Probably an attempt to make a stamp look like what it is not.