You should be able to achieve what you want just by extracting wordpress into /blog on your root, so the working directory would look something like this:
public_html :
config
concrete
packages
blog :
(wordpress install)
(other concrete5 stuff)
Then just navigate to yoursite.com/blog and you’ll be within the correct wordpress dir, then just install like you would normally.
That should work without issue, Concrete5 does a good job allowing that sort of thing.
