The function of a DC-block is to stop the passage of direct current by allowing only RF to flow. Let's take the example of a spectrum analyser, often a probe or simply a cable is connected, which is used to 'find' the RF signal on the plate being repaired, e.g. the OL signal in an RX.
If you do not interpose the DC Block on the input connector of the instrument, you risk damaging the spectrum analyser, since with the RF signal of a few millivolts, the DC (typical +12V) present on the transistor collector will also be present as a power supply, and this could enter the mixer or the analyser input attenuator, damaging it. Recall that a voltage of 12Vdc across 50Ω resistive capacitance has the same effect as as much as 2.9W of RF power (P = V2 / R) to our spectrum analyser.
The performance of the DC Block is to have as wide a bandwidth as possible, with as little ROS and attenuation as possible, so that it is almost transparent to RF alone.
