If I were to mix some of my non-bulletproof inks with Noodler's Blue Ghost, would the new ink be bulletproof? Would the Blue Ghost change the look of the original ink on paper?

The Blue Ghost would not make the other ink bulletproof. Instead, what would probably happen would be that the other ink could be removed, but would leave the (now invisible) Blue Ghost behind. You'd be able to detect forgery or alteration by using an UV light, which would light up the Blue Ghost.

Whether the Blue Ghost would change the appearance of the other ink on paper is an interesting question. If I had time, I'd whip up a couple of batches, but I don't so this is my guess: It depends on the color and the dye content of the non-BP ink, and it depends on how much Blue Ghost you put in.

If you used Whiteness of the Whale instead, your non-BP ink would change color (lighter, maybe even pastel-like, depending on the non-BP color and how much WotW you used), but the effect would be the same. The WotW would resist alteration, and would still glow under UV light. If a forger tried to match the color of the ink and simply over-wrote part of your writing, it would be immediately obvious under UV. In addition, if someone tried to just "fake" your color, without using WotW, it would be easy to spot the fake under UV. I believe this is why WotW was developed. WotW doesn't seem to have caught on, so it is very likely that Nathan won't make a "successor" to it (he can't get the WotW ingredients any more, IIRC).

If Nathan could invent an ink that when mixed with others would provide bulletproof properties, yet keep the color of the original ink, I would buy a bottle and always keep it in stock.