I use Blender for my product design!
The same as any professional. Know your art basics. Know your application inside and out. Be proficient in what you do. Always be learning something new. Understand the technical pipeline of production in your area of work. And most of all, have talent.
Upon a rock an old wise Blender Master sits. A bee enters his personal space and is met by the lightning quick smack of a mouse and the Master wields it from the end of a cord. He strokes his mustache tails and opens his eyes somewhat wider than the usual near shut state.
For one to be-uh-a-master of de Blender, one musht be berry BERRY patient!
I want to know what does a person need to know to use Blender at a professional level.
Easy. You need to take paying jobs (or parts of paying jobs) and complete them up to spec, above expectations, and on schedule.
What kind of paying jobs? Well, whatever needs to be done that you are best at. Archviz? Product rendering? Animation? Motion graphics? Rigging? Animating? Lighting? etc.
You also need a badass portfolio, and if you are a freelancer, you need good sales skills and lots of business contacts.