Will washing a certain part of my blender in the dishwasher ruin it?
Many blenders can be decently cleaned by just blending a couple changes of water, sometimes with detergent added in the first change, in them.

If the manufacturer says the part is dishwasher safe (adhere strictly to instructions about temperatures, detergents, and racks to be used!) and it degrades, get a warranty replacement. 

There are two potentially problematic spots in a blender jar where a dishwasher could ruin things:


Seals.
Blades. 

Some blender designs use sharp blades made like knife blades - such are sensitive to:


mechanical  edge damage (if the blade is allowed to bang against anything else in the dishwasher!), 
thermal degradation (prolonged heating even at 70/80°C can mess up the temper of a blade in the long run), 
corrosion (almost all hardenable knife steel, even if called stainless, is more corrosion sensitive than pot&amp;pan type nickel austenitic stainless steel. If the manufacturer used an extraordinarily corrosion proof blade material like SM-100 alloy or H1 steel, they would for sure advertise it and ask for quite a markup. 


Other blender designs work with rather blunt blades that rely on sheer collision force, such designs could be expected to hardly suffer from the abovementioned effects.