The only problem with that is when you go to submit multiple blogs using that format, say using Twan's Blog Submitter, is that they are going to read as the same blog.
Mydomain.com/niche1, mydomain.com/niche2, mydomain.com/niche3 and so on are simply sites in folders on your domain, not separate websites.
If you want to have different niche blogs on one domain you can use subdomains which would look like this:
http://niche1.mydomain.com/
http://niche2.mydomain.com/
http://niche3.mydomain.com/
That way you can submit them to the same directories, etc. and they will be treated as different sites/blogs.
I found this out the hard way when I started out. Hope this helps.
Most WMs use Wordpress. I suggest you start off with it, it's easy to use and Wordpress has good tutorials for beginners.