Can a specific heat be a negative number?

Specific heat cannot be negative. The specific heat of a substance is the amout of heat necessary to raise 1 g of a substance by 1 degree celcius in temperature. You might be thinking about enthalpy with a negative number, because when a reaction is exothermic, the enthalpy change, or ΔH, is less than zero. You can use the specific heat of a substance of known mass to calculate the enthalpy of a reaction in a closed system (for example, in a bomb calorimeter you use the specific heat of water, mass of the water, and the temperature change of water surrounding a submerged reaction vessel to find enthalpy of the reaction).

With antifreeze you want a negative number if you live in a cold climate so your block doesn't freeze and crack.

it can be, but it isn't for antifreeze. it should be around 2.4.