A blocking capacitor is in series with the AC signal. A capacitor blocks DC current, but passes AC current. You may have learned that the reactance (impedance) of a capacitor is
Z=1/(2*pi*F*C),
where F is frequency. In this equation, note that if F=zero (DC), Z=infinity. Therefore, no DC current can pass. As F goes up, Z goes down. At some frequency, depending on the value of C, Z will be close to zero (relative to the load resistance), so the cap will look like a short circuit to AC at that frequency and all higher frequencies.