I'm a long time BlackBoard/Turn It In user. I am using Canvas for the first time at another university. Generally, they are very similar and intuitive.
However, when it comes to grading papers, there is one GLARING difference between Blackboard and Canvas. It has to do with annotated comment banks.
I agree with you that what is available in blackboard would be nice in Canvas. There are some reasons why that capability is not present - I would like the comments to be like rubrics - where you could assign them to specific assignments like you mention. I have a feeling that there are 2 possible reasons for the difference. One is that Canvas just not develop the saved comment aspect very well - it is brand new, so it may get better over time. The other possibility is blackboard may have a patent on that type of layout system.
I have a method of using the Canvas comment library that might help you:
I have at least two dozen comments in my library (and periodically add more). What I do to identify and quickly find the one I want to use is to start each comment with CAPITAL LETTERS (give it a name, so it stands out to me) that are specific to the assignment and, if needed, specific issues that commonly come up with student work. When needing to find a specific comment in my library, I just type a word I know is in the comment I'm looking for and the option to select that comment pops up so I can just click on it. I can then add to or edit the comment as needed for that specific student's issue. (I will usually remove the "name" I assigned to the comment).
Yes, it is a work-around and not as effective as having specific collection of comments for each assignment. But sometimes I do use the same generic comment as a base for multiple assignments (e.g. "view feedback added to submitted file" or "please use helpful header with your name, assignment title and course number/section..."