Although John Petrucci isn't a straight "blues player", he definitely has some cool blues ideas that convey serious attitude. And hey, attitude is what blues is all about right? :) This particular riff was inspired from some of Petrucci's phrasing in his famous "Under a Glass Moon" solo. I like to throw this lick into my playing when improvising over blues or rock tracks.
We're going to be drawing from the E minor pentatonic scale in this riff. The first note involves a big, bluesy bend. The most important thing with bends is making sure the correct pitch is reached during the bend. In this riff, with the initial bend starting on the 15th-fret B string, make sure you bend up a whole step(2 frets) to where the 17th-fret note is heard.
When practicing bending, it's always good to play the note you're trying to reach before you attempt the bend. This helps make sure you reach the correct pitch. So for this riff, you would first quickly play the 17th fret on the B string (E). Then when you actually play the bend starting on the 15th fret, make sure the pitch sounds exactly like the reference note you just played.
This riff also has a good amount of legato. Make sure your pull-offs and slides sound crystal clear. Also, be careful with your picking. Don't settle for sloppy picking! Use the pickstrokes exactly how they're indicated under the tablature and keep them accurate.
Lastly, be sure to play this lick all over the neck in different keys. Have fun!