Sugar becomes black when comes in contact with conc.${\mathrm{H}}_2{\mathrm{SO}}_4$ It is because of:

The sugar, a kind of carbohydrate like  ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_8$, which is combined with sulfuric acid ${\mathrm{H}}_2{\mathrm{SO}}_4$ Carbon is left behind after the carbohydrate is dehydrated by sulfuric acid, which also removes water. 

As a result carbon foam is produced when bubbles in the carbon are produced by the hot steam and sugar become black