So a couple of weeks ago, my 3ds got this black ink looking thing on the inside of it's screen, and it doesn't harm my 3ds but it blocks some of the screen. How do I get rid of this?
I had, and still have, the same problem with my DS Lite. The only way you're gonna be able to get rid of it is if you send it to Nintendo and have it repaired. Try not to touch the smudge in the meantime.
You'll need to replace it. While I think it's harmless, it's a real pain and I think Nintendo will ultimately send you a brand new/refurbished system. My brother had a similar problem (caused by putting his 3DS under a lot of weight unknowingly), he had to get a brand new one from big N.
I had, and still have, the same problem with my DS Lite. The only way you're gonna be able to get rid of it is if you send it to Nintendo and have it repaired. Try not to touch the smudge in the meantime.
Some of the pixels in your screen have worn out or cracked. This happened to me, I left my school laptop on before and I came back and 25% of the screen had a black smudge on which would expand if you touched it. You need it repaired or the screen replaced.