I posted the link becuase I found your original definition of Neapolitan very simuliar to the one on the link. Not to say you got your information from that link, but somewhere along the line you learned the correct spelling (neopolitan / neapolitan). My point being, that maybe you don't have this stuff as "under your belt" as Jolly suggested, but merely under your fingertips. Of course this is merely a speculation, not an accusation. Anyway, I would be interested in hearing about the Neapolitan scale since I have no recollection of a scale by this name.

Originally posted by Jolly McJollyson
oh, I see! I use something like that when I play blues:
It's like A7-D7-D#7-E7 and the D#7 kind of works the same way as your "neapolitans." Maybe it's the same thing (guess not since it's not a major chord but is, as you can see, a 7th), I don't know.

Everything Notice has stated about the Neapolitan is true, in modern times the Neapolitan has transformed into what is now known as the "tri-tone substitution". It is named the tri-tone sub for 2 reasons: 1) It's a chord that is subbed for another chord whose root is a tri tone away (dim.5th /aug. 4th). It was 1st, and most commonly used as a sub for the dominate 5 chord. In the key of C maj the dom. 5 is G7, a tri-tone away from G would be Db, the Neapolitan.
2) The tri-tone substitution will share the same tri-tone interval (3rd & 7th) as the chord it is subbing for (enharmonically speaking). For ex.: Db7..Db,F,Ab, Cb.....G7..G,B,D,F. The 3rd and 7th of the sub (f & Cb) is the reverse of the 3rd and 7th of the original (B & F). The major difference between the classical version of the Neapolitan and its modern contemporary is in resolution, the Nea would most commonly be resolved to the 5 chord, while the tri-tone is most commonly resolved to the tonic. Common chord progression: Dmin7...|G7...|C...|| same progression w/sub: Dmin7...|Db7...|C...||. Also its modern contemporary does not usually use the sub in first inversion as the Nea was used. This was only originally done so to avoid parallel 5ths in the voice leading, a big no no of that time.
Now, to get back to your original statement about using a tri-tone sub in a 12 bar blues. The most common variation of this would be to insert the sub for the 4 chord, usually in measure 2, but if you wanted a real tense sound you could do it in measures 5 & 6. Examples
A Blues
|A7...|D7...|A7...|....|
|D7...|....|A7...|....|
|E7...|D7...|A7...|D7.E7.||
most common used tri tone sub in a 12 bar:
|A7...|Ab7...|A7...|....|
|D7...|....|A7...|....|
|E7...|D7...|A7...|D7.E7.||
If you really wanted to push the boundaries
|A7...|D7...|A7...|....|
|Ab7...|....|A7...|....|
|E7...|D7...|A7...|D7.E7.||