Correct me if I'm wrong, but your blood pressure of 120/90 is the systolic over the diastolic pressures. The 120 is the pressure exerted by each heartbeat (outpump), and the 90 is the pressure maintained by the tone of the viens and artieries in between heartbeats.
A high diastolic means there is a lot of tension in the viens. Whatever the cause it is undesirable, as it means you are expending more
energy then necessary to bring nutrients to each part of the body. A lower systolic/diastolic is desired, because this means your heart needs to work less due to a more oxygen and nutrient enriched blood. Less pumps are required to bring an equal amount of nutrients to the body.
This is a generic statement of course. It's possible to have low blood pressure problems as well. For althetes however, it's the lower blood pressure that is desired.
Vasodilation is an expansion of the viens, not a contraction. This would result in a lower diastolic value, and possibly a lower systolic value. I'm not well versed on the effects of NO2, but if it is a vasodilator I would assume it would lower your blood pressure values.
The heart and veins will never "atrophy" or weaken from lower values. It is desireable for greater athletic performance.
-Lite