A trademark is a symbol and/or word registered by use as representing a company or product. It is intended to identify a specific source of goods or services, protecting that source from confusion in the minds of consumers with other sources.
The word Arduino and logo (eternity eight symbol) is a trademark of Arduino SA. This means it can not be used by any third party on either products, in domain names, email addresses, in names of web pages, social media, blogs and more.
However since we want to make sure people can identify projects, initiatives and activities coming from the passionate community around the globe, self-organizing local activities and/or online content focused on Arduino we have created the Arduino Community logo, which you can use as long as you run a non-commercial blog.