A Group Blog exists for users to share their thoughts or information on a topic. it’s an article or editorial that allows for comments, whereas a group discussion is intended for multiple participants. Blogs can be an opinion piece, can include a call-to-action or act as an introduction, providing information about the group.
See How so I post a blog article for more details
A Group Discussion is intended to be an asynchronous conversation between GCcollab group members. You can receive answers and gather the opinions of members. Any member of a group can post a discussion topic, give context, or ask questions. Group members can reply to the original discussion post, or to comments made by others in response to the original post.
It is important to note that users receive notifications on the first post in a discussion thread only. If they wish to receive subsequent notifications, they must subscribe by clicking on the bell icon for that thread, or by contributing to the thread.
See how do I post a group discussion article for more details.