In both the major and minor scale, the 5th is almost always a major dominant chord. In the major key it happens naturally, in the minor scale it is taken from the harmonic minor scale. The only time it is minor is if the melody descends from the tonic to the 5th. In the rules of the melodic minor scale; ascending its #6 and #7, descending it is natural again. The minor dominant chord is rare because the #7 moves to the tonic in the progression V-i. The #7 is the 3rd of the dominant chord, and it is major so the chord would make sense if it was a major chord to. Sometimes the melody descends when the 7th is natural again, changing the dominant chord to a minor.

When using them in context. Blues is done by using major chords, and imposing a minor scale on top. Producing the Blues minor and blues major scale. You can have minor chords, I'm not saying they are not used. Just find a chord progression that sounds bluesy in your own mind and then play a melody over it. Like I said it the other thread, theory can't make great music for you, that's up to you and what you want to hear.