Does a capacitor block ac current dc current or both?

No you can't use DC capacitors in Ac because in DC current , the current flows directly while in AC current, the current flows curved zig-zag. Thus Dc equipment can't mingle with AC as they need Direct flow. So DC capacitors can't be used for AC current...................

Most capacitors will operate on either DC or AC. The exception is the polarised-type capacitor, which is designed to operate with a DC or an appropriately-biased supply.

Often, a capacitor's rated voltages is specified in terms of DC -for example, '100 V (DC)', so care must be exercised if it is to be used with AC. This is because we always measure AC voltages as root-mean-square (rms) values, not as peak values. For example, '100 V (AC)' actually peaks at 141 V*. So, applying 100 V (AC) to a capacitor rated at 100 V (DC) may cause its dielectric to fail. In this example, the maximum allowable AC voltage must be 70.7 V, which peaks at 100 V.

(*To explain: Vrms = 0.707 Vmax )