Answer:
First of all, when a company forbids its employees from blogging about company products it does not violate the first amendment. Employees are required to keep confidentiality about the company they work for.
Many companies prohibit employees from blogging (or any use of social media) about the company because employees represent the company they work for. E.g. imagine that you work for AT&T. If you make a bad comment about cell phone services and other people know that you work for AT&T, it may end up damaging AT&T's reputation or brand image. All types of videos and postings go viral, and even comments that you make to a friend might end up being seen by thousands or millions. You are free to express any opinion that doesn't affect your company, and there are thousands of subjects you can talk about. Just do not talk about work.
