Hi there! :)
Someone who is dehydrated has below-normal levels of water in the blood.
Blood that is hypertonic has a much higher concentration of dissolved solutes in comparison because of lower levels of solvents, or water in the blood.
In general, a hypertonic solution has a much higher concentration of solutes compared to a liquid inside or outside a semi-permeable membrane. Therefore, if one is dehydrated, their blood would have a much higher concentration of these solutes in compared to the amount of solvent (water) which would deem it hypertonic.