This seems to be an ongoing question that I perhaps should have been more clear about in the lessons. Like Christopher said, whenever we are dealing with a chord progression, traditionally we use a Roman Numeral system that allows us to see the chords in a different way. This allows us to be able to change keys and to also "hear" how all the chords relate to one another. Here is how that concept pertains to those lessons:

12 Bar Blues: Minor Blues in D Minor w/ Flat 6 Chord
- this progression is in the key of D minor. Here are the notes of a D minor scale and their appropriate scale degrees.

I - D minor
II - Emi(b5) - or half diminished
bIII - F
IV - G minor
V - A minor
bVI - Bb
bVII - C7

If you look at this, you can than see how the 12 Bar Blues is constructed. Here is how our 12 Bar Blues in D minor pans out;

I - D minor
IV - G minor
V - A7 (we use this instead as it works better with the blues tonality)
bVI - Bb

12 Bar Blues: Minor Blues in E Minor w/ Flat 7
- This lesson works similarly. Here are the notes and chords in the key of E minor.

I - E minor
II - F#mi(b5) - or half diminished
bIII - G
IV - Ami
V - Bmi
bVI - C
bVII - D7

Here is how our 12 Bar Blues in E minor will pan out;

I - E minor
IV - A minor
V - B7 (we use this instead as it works better with the blues tonality)
bVII - D

As you can see, the bVII chord in this key is D7. In this lesson we use simply D major, but the same idea applies. This song is actually a similar idea used in a John Coltrane song entitled "Mr. PC." That is a very common blues/jazz standard that showcases a unique minor blues variation.

We are simply pulling those chords from the minor scale. It is important that you grasp how major and minor scales are constructed in order to grasp this idea. That knowledge will aid you in many, many things in your musical life. Feel free to keep asking if you need further clarification.