It’s free and can do all the things that BlackBoard does, only–at a cost of your professionalism.
The main difference between the free Moodle platform and BlackBoard is that in BlackBoard all functionality is predesigned whereas in Moodle one needs to investigate on how to do things. For example, tables. Moodle is bad at doing tables and it’s easier to avoid them altogether, whereas in BlackBoard they are readily there at one’s disposal. There’s a very good article that compares Moodle and BlackBoard which shows what both platforms can do in comparison. The main argument there is that BlackBoard saves time and money (!) in the long run. Maybe this article was sponsored by BlackBoard?
Let’s not compare anymore. What is good about Moodle besides being free? The main thing for me is that it allows to create whatever you want as an educator. You start with a barren place and you build on it the components you want to depending on the course you’re teaching. It helps a person to construct things that no one has done before and, thus, experiment with learning facilitation. Only, when you add students to the class, sometimes you need to provide detailed description of what to do and where to go (unlike Blackboard, which is kind of intuitive).