What features does a breakfast or a morning blend have that other roasts or varieties don't have?
Is it always the similar or does it vary by distributor?
Is there any difference between breakfast and morning blend?
Breakfast or morning blends are generally a lighter roast, but there is no industry standard for how light of a roast.
As for differences between breakfast and morning blends, the key is in the word "blend". It is entirely up to the producer to blend different beans and different roasts and call it what they want.
Edit: There are some differences in caffeine levels, but a lot of contradictory information out there. I found a great explanation of why the caffeine levels can differ between roasts here, but let me summarize from the post:
If you measure your coffee by scoops, light roasted coffee will have more caffeine. Since the beans are denser than a darker roast. However if you weigh out your scoops, darker roasts will have more caffeine, because there is less mass.
@ChrisAK is correct, the roasting process does not burn or destroy caffeine, but the level of roast will impact the weight of each bean.