A connector block for retaining at least one shock tube in signal transfer relationship with a detonator. The connector block comprises a housing (2, 21) having a bore (4, 23) formed therein for receiving a detonator (6, 36) provided with a percussion-actuation end (7, 25), a shock tube retention means (8, 28) defining with the housing a slot (9, 29) for receiving therein at least one shock tube (10, 30) and holding the at least one shock tube in signal transfer relationship with the percussion-actuation end of the detonator present in the bore, the slot having an entrance (12, 32) for allowing insertion of the at least one shock tube into the slot, and a flexible and resilient closure member (11, 31) or an inflexible closure member (50) pivotable on a sprung hinge (51) extending partially or fully into the entrance. The closure member and the shock tube retention means resiliently flex to allow entry of the at least one shock tube through the entrance and into the slot, the closure member flexing through a distance at least 30% the diameter of the shock tube.