Again, the rule applies: Whenever designing for print, think inks, not just colors.
For example: If your "black" contains just K ink, and your "red" just contains M and Y inks, then the two "colors" do not share any component inks. This very often causes grads (including meshes) and even blends to look muted and to suggest an appearance as if there is an intermediate white or grey.
Think what's happening in terms of inks. The black ink (K) starts at its color stop in the grad and diminishes to 0% where the M and Y inks only start to appear. So it looks almost like (especially in print) there is an intermediate lightening where you don't intend any.
The fix is to include a significant component of the other "color" in the "black." In the case of a "black-to-red" gradient, try something like 100M, 50Y, 100K for the "black." That way, there will be a significant percentage of a darker ink (M) to "bridge the gaps" in the transition areas.
JET