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.

To understand..You want to change your domain to a new one? And you currently have a subdomain like myblog.blog.com ?

Hello,

If you will be focusing on some sort of cosmetic products on your "primary domain" that will be ok, and the also the subdomain.

But if you will be blogging about cancer, I suggest you create a new domain for it or subdo.

Don't mix in one domain cosmetics topics and cancer topics not unless it is a general blog with a lot of different topics because for your if your blog or domain is set for SEO, google will crawl it.

Better if you have focus on what you're blogging.

Thanks!

If you "blog" regularly about "Off topic" items in your main setup, you'll simply be diluting any SEO efforts.

Domains are cheap - just buy another one for your cancer blog

Especially with the 1.5-2USD domains you have those days...Just better get one more domain

$2 USD domains?
A good .com/.org/etc is generally in the $10 range.

Domain name is not really important for SEO

You couldn't be more incorrect.

A domain name is perhaps one of the MOST important factors in SEO. A well-chosen name is the start of your relationship with search engines. Everything from domain name age to keyword(s) plays a vital role in how search engines will rank it.

I believe they were referencing other TLD's such as .info, .biz etc.. They're usually in the $2 range

To clarify, you have:
domain-about-cosmetics.com
blog.domain-about-cosmetics.com

You're looking to write some content regarding cancer and paste it on the "domain-about-cosmetics.com" location - is that right?

If you're going to hit the niche hard, then you might want to get "cancer.domain-about-cosmetics.com" or other domain specifically for the cancer topcs and then cross reference them.

Putting content on your existing blog about cancer should not negatively affect your current ratings for your existing keywords on your site. Adding more content targeting a different keyword/niche (cancer), would just mean more exposure of your domain to a different audience.

They cost not more than $2-3.