Should I block my basement air returns to better cool my top floor?
Heating and air conditioner units have a fan that pushes air and it has a maximum ventilation capacity. The air return “allows” the air conditioner to draw air back into the system and is called “make-up air”. 

That is to say, the fan can only “push” so much air, whether you’re heating or cooling and your unit will draw the same amount of air back into the system. 

If you are “cooling” the basement too much and not cooling the upstairs enough, then you want to distribute more air to the warm areas. This is called “balancing the system”. 

Without re-sizing all the ducts, you could cover the “supply” vents in the basement, which will force more air to the other areas. (You may be able to just partially close the downstairs vents, if you have adjustable vents.) By redistributing that air to other areas, those areas my get too cold, so you’ll need to “balance” the system by adjusting those vent covers. 

During the cooling season, we like return air vents where we can bring cool air into the system and re-cool it to the desired temperature. (Likewise with the return air for the heating season, we like to have warm air returned to the system so we can re-heat it.)

I doubt if you have 2 return vents, I’d just cover some of the supply vents in the basement until it’s comfortable. 

BTW, by covering some vents, you are forcing more air into other ducts. If those ducts are too small to “handle” the additional volume, you’ll hear a slight “whistle” sound. If so, you’ll just have to re-adjust how many vents you cover in the basement.