Firstly, Earth is very proud of its blue and green color, if Humans painted Earth Black, Earth may not be able to show off anymore.
Secondly, if Humans painted Earth Black, black color is known to absorb most heat, people may keep on sweating profusely.
Thirdly, if Humans painted Earth Black, everything will look black now, thieves may have a gala time.
Fourthly, if Humans painted Earth Black, even Aliens may paint themselves black and start living with us.
Fifthly, if Humans painted Earth Black, Moon may paint itself black in order to cheer up planet Earth.
Lastly, if Humans painted Earth Black, couples who are just about to get married may not recognize each other anymore.