For those blogging for charities based in the United States, your charity must be a registered 501(c)(3) charity. This information is usually readily available on your charity's website, probably in the donation section; if you can't find it, call or email to verify their status. If the organization you are blogging for is based in the United States and is not a 501(c)(3) (for example, if they are a 501(c)(4) instead), you may not blog for them with Blogathon.
For those blogging outside of the United States, we strongly encourage you to blog for an organization registered as a charity in your country. If your country does not have a charity registry, you may still blog for your organization provided they are a charitable organization. For the purposes of what counts as a charitable organization, we'll refer to Dictionary.com, who say that a charity is:
a foundation created to promote the public good (not for assistance to any particular individuals)