How can I use Blackboard to give students feedback on their assignments?

The Blackboard Learning Management System has three primary functions: Content Delivery, Communication and Collaboration and Student Assessment and Grading.
When it comes to student assessment and grading, Blackboard includes tools such as Inline Grading and the ability to attach and files as assignment feedback so that instructors can give detailed critique to student work. Additionally, the Turnitin Building Block allows instructors to have students upload assignments to Turnitin that can produce an originality report as well as text and audio comments on student work.
The journal tool provides for a simple way to provide for a private conversation between a student and the instructor within the course space. An important point to remember is submissions in systems such as Blackboard and Turnitin are time/date stamped helping to track and archive student work.

Additional Resources:
Creating a Turnitin Assignment in Blackboard
Adding Voice Comments to a Student Submitted Assignment

Which Blackboard tool can I use to create collaboration among students?

Blackboard has a blog feature and a wiki that can be used to create collaboration among students.

A blog is a personal online journal where an individual can record opinions, and link to other sites.  A blog is intended for public access and use.  The Blackboard blog feature is limited to public access within the Blackboard course.  Blogs have a commenting system that is used for responding and interacting with one another. Blogs can encourage students to express their ideas. Blogs are an effective tool for gaining insight into students’ activities and provide a way to share knowledge and materials.

Examples:  Students can choose or be assigned a topic to write about in their blog.  There are three types of Blackboard Blogs: Individual, Course, Group

A wiki is a website that allows collaborative editing of its content and structure by its users.  Wikis are online databases, collaborative collections of information that uses interlinked pages to present and organize data