Primary domain..

Let me start of by stating what I want, I want my domain to have a blog of its own , as I already have a domain and a same sub domain. Basically I am using good keywords in my primary domain and the contents of it are cosmetic products. Would I have a different niche while rooting for my domain for my new blog? As I don?t want to have any kind of problems with SEO issues in the future, I was asking that will it be bad for the site if I blog about cancer on my site.

I think it will be bad. It would be much better to have different kinds of content on different sites, and this is better both for your users and the search engines.