The 10th Doctor’s blue suit jackets’ backs all appeared to be cut with the center back seam on a pinstripe.
Like the brown suit, the blue suit had a tapered back, as tailored jacks often do. (That is, the center back seam line was taken in via a gentle curve moving downward toward the waist and small of the back.)
The effect was more easily observable on the back of the brown suit (both versions):
However, note that the tapering was considerably less apparent on the blue suit – to such an extent that the back often didn’t appear to be tapered at all!
This was partially due to the subtlety of blue suit’s pinstripes, which were often difficult to see.
They were also farther apart than the pinstripes on the brown fabric, but one can nevertheless observe that the blue suit’s back was tapered:
The lower back of the 10th Doctor’s blue suit jacket had a single vent at the center back, with the back left overlapping the back right.
The back waistband was positioned slightly higher than the “faux-flap” pockets.
At the top of the back vent was a waistband, which was self-fabric cut on the crossgrain (i.e. the pinstripes were horizontal). It spanned the entire back, from one side/back panel seam to the other.
The brown suit’s back waistband was simply edge-stitched onto the back of the jacket body along the upper and lower edges, so in lieu of any evidence to the contrary, I believe the blue suit’s was constructed in the same manner.
I’m not sure if the back waistband was a consistent height on every blue suit, but the one time I got a clear glimpse of it in relation to the horizontal pinstripes, it appeared to be four (horizontal) pinstripes tall.
This back waistband was a nice touch for a suit based on one from the 1930s.
You can see similar back waistbands on other garments in projects which take place during that era, such as Indiana Jones’ and Belloq’s jackets in Raiders of the Lost Ark (which took place in 1937):