You can certainly take pictures of people in public because they have no expectation of privacy when they are in public. You might even be able to post them to your blog, provided your blog is non-commercial. However, once you make money on your blog/commercialize it, you may have violated the right of publicity of those individuals who appear in the pictures and they may have a right to monetary payment for your commercial use of their images. Generally people have exclusive rights to their name, likeness, and image which can't be commercially used by others without either a waiver or payment. That is why model releases are almost always used by commercial photographers.
An example of a model release is provided by way of the link below.
No, not if they're in public, since that's a place where no one expects any privacy. As my colleague noted, however, you'd be violating their "publicity rights" if you exploited their images for money without their consent, since everyone (not just famous celebrities and athletes) owns a property right to their own name, likeness, voice, signature, etc.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
It may be nicer to obtain permission first. Depending on the particular details of the pictures, some people might object to being portrayed in an unflattering light. Being nice tends to be good for business.
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