We have a number of customers who are running Digitrax alongside their existing block detection circuitry. There are numerous block detection schemes and circuits, and we cannot cover all the variations here. Generally, it is necessary to isolate the block detector power supply and ground system of your existing block detector system from that of the Digital boosters. The most common detectors use "back to back" high current diodes to sense block current-draw. Detectors with "opto-isolated" or relay outputs will generally have no problem sensing current draw of a loco, etc., on the DCC current, and will signal the occupancy without any ground or power supply conflicts. Many DCC boosters, including Digitrax, work with both rails live and a separate system ground wire. This ensures no possibility of booster output voltages being "added" accidentally across adjoining booster power districts, and operation on catenary is possible.
If your layout is common rail, all you need to do is be sure to double-gap the layout between power sections supplied by different boosters.
Within the "power district" of a single booster you CAN have a common rail return between detection blocks as long as all the detectors are within the same booster and the detector power supply is isolated from other detectors and booster grounds. If you are not sure, simply disconnect the detector power supplies and reconnect them in stages while running the layout. If there is a conflict it will generally short-out one of the booster outputs and shut down. This will allow you to isolate the problem ground or power supply.