[Plugin: wpDirAuth] Anyway to limit access to some blog only with MU usage ?

I’m not sure I’m following what you are trying to do. All blogs in MU share a common user-base, but each blog can specify what users have access to the blog. Give the version I just sent you a try and see if that addresses this issue. In the new version, you can pre-add (meaning the user doesnt have to login with their AD credentials before you can manage them in wordpress) users assigning them to specific blogs with permissions all at once.

I try to have a private blog among the other. This private blog should only be accessible to certain AD users after login.

I’m testing https://wpmututorials.com/plugins/network-privacy/ which seems to do what I need.

wpDirAuth just authenticates a user’s user name and password against LDAP; authorization to a specific blog, and any permissions to that blog, are still handled by wordpress. Having said that, I did add some pieces to allow you to handle setting authorization at the same time as identifying an LDAP authorized user, but the two pieces are really separate.