Yes the dominant 7 is the flat seventh added to a major chord. Major seventh isn't used much in blues but sometimes it can make things jazzy. Listen to Stormy Monday by the Allman brothers. They play a Gmaj7 before they do the minor walkup. Diminished sevens are neat chords. Any note in the chord can be the root. They add a lot of tension. It can sound cool to play a dim7 one step below the chord you are leading into. For instance if you are playing in the key of C going from C7 to F7 On the last beat or two of the bar before the F7 play an Edim7 (E is one step or one fret below F). You can also play one step up from the chord you just played. e.g. | C7 C7 Edim7 Edim7 | F7 F7 F7 F7 | or | C7 C7 C7 C7 | C#dim7 C#dim7 Edim7 Edim7 | F7 F7 F7 F7 | Then you get into sixth and ninth chords. The blues is all about adding tension and releasing it. It can get crazy trying to figure out all this. I mostly just stick to seventh chords trying to weave into the space between the bass and drums. In a blues jam there is always room for a good rhythm player or even two. Lead players don't get to play a lot because everyone wants to play lead. Many lead players don't practice rhythm so when they are not playing lead they don't know what to do. I spend more time practicing rhythm to jam tracks than lead.