That doesn't strike me as a very clear distinction. If you negotiate for more money in return for remaining silent, is that blackmail?
"Hey thought you should know, my 'security' company is currently for hire for businesses around town. We heard through the underground grapevine that a lot of folks may lose some product in their bodegas next month. Anyway, nice to introduce you to my 'security' business -- have a great month!"
If you revealed that your true intent was blackmail, and that's what this is, to your lawyer or anyone else then I imagine the intent in combination with the act is enough to nail you.