This is sounding like it may be a tritone... which is basically the maj 3rd and min 7th of the chord.
A major chord consists of the root, maj 3rd and perfect 5th of the scale.
By adding the 7th to the chord we get '7th' chords, which can be maj 7th, min 7th or dom 7th. Blues often uses dom 7th chords... typically the I7, IV7 and V7 chords, for example C7, F7 & G7 in the key of C.
When adding the 7th [F] to the V chord G [GBD] we get a G7 [GBDF] - Dominant 7th.
For the I & IV chords, when we add the 7th we get Maj 7th chords, rather than dom 7ths. To make them Dom 7th, we have to flat the 7th. So for the I & IV chords we add the minor or flat 7 to the major chord.
So we get
Dom 7th Chords - Notes
I - C7 - C [root], E [maj 3rd], G [5th], Bb [min 7th or flatted 7th]
IV - F7 - F [root], A [maj 3rd], C [5th], Eb [min 7th or flatted 7th]
V - G7 - G [root], B [maj 3rd], D [5th], F [min 7th] - Natural occurring Dom 7th
The 3rd and the 7th notes are KEY notes, as the 3rd establishes the chord mode[minor or major] and the 7th establishes the type of 7th [major 7th, dominant 7th for major chords [minor7th for minor chords]].
If you take a dominant 7th and reduce it to the maj 3rd and min 7th [the 2 KEY notes], it will have a dominant 7th sound. This is a tritone and can sometimes be used in place of the full chord.
Here is an example...
In a I IV V blues in the key of C
The tritones are
I - C7 - E [maj 3rd], Bb [min 7th or flatted 7th]
IV - F7 - A [maj 3rd], Eb [min 7th or flatted 7th]
V - G7 - B [maj 3rd], F [min 7th] - Natural occurring Dom 7th
So in playing the tritones on the 3rd & 4th strings you get...
C Tritone - E [4th string 2nd fret - maj 3rd] Bb [3rd string 3rd fret - min 7th]
Sliding the C tritone 1 fret LOWER makes it an F tritone...
F Tritone - Eb [4th string 1st fret - min 7th] A [3rd string 2rd fret - maj 3rd]
because the intervals are inverted... the 4th string note [interval] changes from a maj 3rd of C to a min 7th of F, while the 3rd string note [interval] changes from a min 7th of C to a maj 3rd of F
But wait there is more!!!
If you slide the C tritone 1 fret HIGHER, you get the G tritone
G Tritone - F [4th string 3rd fret - min 7th] B [3rd string 4th fret - maj 3rd]
This is equivalant of sliding the F tritone up 2 frets to become a G tritone.
I hope this helps and doesn't give anyone a headache! ;-)