I've been doing a lot of work like this lately. My suggestions are basically in line with what others have already said.
for the black shirts:
choke in your underbase using curves.
expand the glow using curves so it covers the base and about 20-30% more.
This will depend a lot on the color being printed and dot gain etc. I recommend testing if possible by making a square, applying a gradient from 100-0 on it, duplicating, then adjusting the curve as suggested (down for base, up for glow). Do this to different extremes, but tile them on the same two screens. Print and see what gradient combination looks the best.
For the blue shirt:
use two screens if you want the best outcome, one for the solid black, one for the gradient. Bring down the curve of the gradient AND adjust it so there is no solid black. I would start around 75% (depending on your dot gain on press). This way you can print the gradient with whatever pressure produces the best results while maintaining a rich solid black. If you have your crap dialed in, you can do this with one screen, but it takes a lot of control over variables.
I would suggest using a discharge base or even two bases on the black shirts so you have finer control over it with less gain etc. I would also base down your top colors to produce more opacity, which will aid in smoothing the gradient over the unbased areas and into the black.