How black hat is this method and can it get me into trouble??

Ok, lets say I have a website about Rihanna. What if I created a post on my blog about Rihanna that said "submit your email here to get Rihanna's s3x tape free"... Even though the s3x tape doesn't even exist.

Could I get in trouble by Rihanna's corporation/agency?? Or from my ISP for posting something like that...

But lets pretend it's in a niche that doesn't have anything to do with s3x, and I am just offering customers something that would really help them but doesn't even exist like a video game-addon or plugin for cheat codes.. Could I still get in trouble from my ISP or the video game owner?

I figured doing this the right way and with a popular "niche" could get lots of good traffic in search of something.

Doesn't that fall under the banner of false advertising?

If you aren't charging for it, I don't think you can get into that much trouble. Although, you are better off grabbing emails, and giving them something of value.

Over here in the UK, contract law states that if you promise to give something in return for something and don't, you are in breach of contract.

it's just not worth it.. honestly, i'd just find some black chick name who's not actually Rihanna, but looks like her... see where i'm going?

Depends what monetization programs are you using. As legal trouble , don't worry , however sometime trademark law may affect you and some ppl will want your website down.