I use piwik on maindomain, for example: www.domain.com
Now I have a new subdomain, for example: blog.domain.com
Should I installing piwik in /blog or can I use the piwik installation on the maindomain?
You can use one piwik installation to track multiple sites. You have to add the new site under settings > websites. This will add the site to piwik and generate a new tracking code for this site.