Display User Author Details in Block UserProfile
The alias doesn't matter, you get the argument for a contextual filter from the internal path, no matter if the URL in the browser was aliased (unless you enable the option "Use path alias"). For a user profile the internal path is /user/[uid]. 

For the user ID there is also a contextual filter "User ID from route context", this filter is able to get the user without specifying in which path position it is.