As you may recall, the 10th Doctor’s brown suit trousers were “off the rack” GAP trousers, not custom-made costume pieces.
However, neither the brown or blue suits’ trousers were ever prominently featured in the show, and I don’t currently own or have access to a pair of screen-used blue suit trousers for comparison.
Considering the overall design similarity between the two suit jackets, I believe it’s safe to assume that the blue suit’s trousers were basically the same style as the brown GAP trousers – perhaps even patterned directly off of them.
The blue suit trousers certainly appeared to share all the major characteristics I mentioned in my 10th Doctor brown suit analysis.
The 10th Doctor’s trousers were “low-rise,” consistent with modern fashion trends (as opposed to high-rise, Victorian-style “grandpa trousers” that many of his predecessors wore).
At the top of the trousers was a sewn-on waistband.
On the brown suit trousers, the waistband was cut on the crossgrain (i.e. the pinstripes were horizontal rather than vertical) and overlapped the right side by a couple inches or so.
In lieu of any evidence to the contrary, I believe this was the case on the blue suit trousers as well – although this is admittedly speculation.
The brown suit (GAP) trousers had seven self-fabric belt loops around the upper perimeter of the trousers that provided the wearer with the option of wearing a belt (although, as best I could tell, Tennant never actually did): two at the side front, two at the sides, two at the side back, and one at the center back.
The blue suit trousers also had belt loops, which appeared to be self-fabric, although I never got a reliable count:
The trousers closed in the center front via a zipper fly closure.
The trousers had slant pockets (also called “yoke pockets”) on each front side.
On the back of the trousers was a double-welt pocket on each side.
A thin loop extended downward from under the top welt around button beneath the lower welt, to secure the pocket closed.
The lower pant legs were blind-hemmed – no cuffs, topstitching, straps, vents, or other funny business.
While the blue suit trousers appeared to be stylistically consistent with the brown suit (GAP) trousers overall, there was one major difference!
The blue suit trousers had two rows of topstitching, on both the side/outer leg seam and inseam! (A flat-felled seam, perhaps?)
While one would probably never notice this particular detail in the show (even on Blu-Ray), it’s quite easily observable in many publicity photos.