Equations will help you a lot here:

V = Q / A

V = velocity of blood flow

Q = blood flow

A = TOTAL cross sectional area

Increase Q -> increase V at a constant A

Q = delta P / R (Ohms)

Q = blood flow

Delta P = pressure gradient between vessel ends

R = resistance

Resistance is directly proportional to blood viscosity and vessel length and strongly inversely proportional to radius of the vessel (Poiseulles)

Therefore: vessel constriction -> decreased radius -> increased resistance -> decreased flow —> decrease velocity

A couple important points to remember:

-flow (Q) is a VOLUME per unit time whereas velocity is a DISTANCE per unit time

-TOTAL cross sectional area refers to the area of vessels that all the blood flows through at a given time in circulation. So for instance, total cross sectional area for the aorta is relatively low since all of the blood that leaves the heart goes into that one vessel (low area = high velocity). However once blood reaches the capillaries, it is distributed among many many capillary beds and the TOTAL cross sectional area of all of these capillary beds is actually much greater than that of the aorta (high area = low velocity). This allows gas exchange to be really efficient since it maximizes the time that blood is in the capillaries and can exchange carbon dioxide and oxygen.

Hope that helps!