Yes, it is an extremely wise decision, for several reasons.
The most important one is that the people who will access and use the blog and those who will access and use the forum only partially (will) coincide. These two different communication channels should be kept separated, because they have different purposes and they will be used by different people.
For example, you shouldn’t assume that a user just passing by the blog will like or accept the idea that he/she needs to subscribe to a discussion forum just to quickly comment on a blog post.
More generally, it’s never wise to force people to move from a more open and accessible channel to a less open and accessible one. You’ll just loose interactions.
It’s OK to invite people to visit the forum and it’s OK if a forum user will decide to open a new thread about a new blog post, but I would never suggest to close a well established and useful communication channel nor I would suggest to automatically create a forum post for each new blog post.