A7 is the "I" chord in an A blues, but if you think about it, it is a dominant chord and dominant chords can't really be "I" chords except in the sense that it is home for a blues, they are "V" chords. Now you tell me; what key is an A7 chord from? It certainly ain't A major so what is it. Think about it before continuing on........
Did you get it? A7 is the "V" chord of D major. So the scale you need to be harmonizing is D major. By the way, what is a D major scale played over a A7 chord? It is the A mixolydian mode right? So you need to be thinking in A mixolydian/D major.
What are the chords in D major from the "V" chord?
A (V)-Bmin (vi)-C#dim (viio)-D (I)-Emin (ii)-F#min (iii)-G (IV)
or A7-Bmin7-C#min7b5-Dmaj7-Emin7-F#min7-Gmaj7 if you prefer to think in 7th chords.
so these are your arpeggios for an A7 chord and all of them can be superimposed over the chord to various degrees of success. Obviously the A triad is fine and the C#dim arpeggio looks like the top half of the dominant chord as it is basically the 3-5-b7 of the chord. The Emin arp is the 5-7-9 of an A9 chord as well. Even an ascending line using all of them diatonically will work fine.
BUT.... You will have to do this with each chord in the Blues progression. The D major key arps will not work over the D7 chord or the E7 chord. You will have to harmonize from G major (D mixolydian) and A major (E mixolydian) for those. Mixing them will be a lot of work but in combination, it could be pretty impressive.