I was thinking to create a blogging platform something like blogger and to allow users to create blogs. My question is.. if a blog will copy content, do blackhat or whatever illegal things and get a penalty from google, all other subdomains (blogs) will get a penalty or the same penalty?
The main domain won't be penalized unless it links to the subdomain (I believe). If the main domain is penalized then all of the subdomains will be penalized. Subdomains cannot penalize each other unless one links to another (In which case the one linking will also penalized). Hope that made sense.
I remember a hosting company offered free subdomains and hosting and after a while someone said that all the subdomains got a penalty from google becouse of duplicate content. 50% of the subdomains had only a default phpbb forum installed. Any other optinios?