some blues is just a riff that gets reapeated like "Midnight Rambler" or "Smokestack Lighting." In Midnight Rambler there is an intro riff, a rock and roll part, and a rubato part with no strict tempo or feel and these are just put around the song. Not all blues has to be a 12 or 8 bar form although the 12 bar form is extremely common.
Hope this helped