posting them or using them for blackmail is most definitely child porn

No, that's considered blackmail, which is generally illegal under a different statute.

What law are you referring to that differentiates whether something is considered child porn depending on the context? I've never heard of any such statute. A photo either is or isn't child porn, full stop.

The same principle holds true in law for other objects as well. A firearm may be legally licensed and owned, or perhaps its illegally modified and unregistered. If I pick up that gun and shoot you, I have committed the crime of murder. I have not changed the legal character of the firearm.