The master calendar approach works, but the real pain point isn't just tracking due dates - it's staying on top of which platform actually has the submission link or if an assignment's been graded. I'd combine that with browser bookmarks or a simple spreadsheet that maps each course to its system, plus a weekly routine (like Sunday night) where you log into both Canvas and Blackboard just to visually scan for anything new. Takes maybe 10 minutes total and catches stuff the manual calendar entry might miss, like discussion posts that don't have hard deadlines or pop quizzes.
Also set up notifications differently on each platform if they let you. Some people turn everything off and rely solely on the calendar, but I'd keep at least one notification channel active per system - maybe Canvas alerts go to your email while Blackboard uses an in-app notification. That way if something slips through your weekly check, you've still got a backup. The key is making the notifications work for you instead of against you, which means they need to be distinct enough that you actually process them instead of ignoring the noise.