Why are you drowning yourself on a cup of water ? Sign in whichever colour you prefer: black or blue. What people are giving here is just general advice for all IRCC processes and past experiences.
For example, nowhere does it say that you shouldn't print double sided, yet there are many cases of people printing double sided and then the agent won't read the verso and request additional documents / mark application as incomplete
In the end, you either take our advice or do whatever you want, no one is forcing you to sign with black or blue.