Integrating blog and customer data on OC

Hi, I have a lot of work to do with product entry at the moment but this has been playing on my mind, I don't want my customers to enter details twice on the site and am wondering if I set up a sub domain for the blog, for example blog.wavecrestscuba.com how I would go about telling whichever blog I use to look at the customer data of OC if you see what I mean??

hi peeps, Does anyone know what it is called that I am looking for here, getting a blog set-up to tie in with reading the customer log-in details so they don't have to sign up twice? I did try searching for things off the site but don't think I found what I need, a nudge in the right direction would be appreciated.

To explain what I want to do, we discussed it and it has been decided that ontop of everything else I am doing, the site will have a blog, people that dive with us can log in and leave a write up of the dive site and their day. Obviously it would be better if they had an account, and getting it associated with some of the details from the account they created for the shop would be great, having the ability to create an account on either the blog or the shop and them working together would be better. If you know what I mean.