What is Blackmart, and is it legitimate? I have an HTC One X. I want to install the Blackmart app, but I'm a little wary about its implications. Can it safely be installed without it messing up root security? Is it a legitimate app source?
Even if the pure Blackmart App is free of any malicious parts, there is still the danger that you will download an infected version. Blackmart has no official page or download site, that's why you can't be sure about that. It's even worse because it needs root. Those types of programs are usually not open-source, and therefore no one can review the code or even build a "clean" version. 

Note: Since Blackmark is able to bypass the access restrictions for paid Apps, it's is illegal in some countries. This is likely true in all countries where Google sells Apps on the Play Store.

My personal opinion: A free game of Modern Warefare or GTA isn't worth the trouble and risk of using blackmart.