Blue ,in the US military usually denotes training, but not necessarily INERT, the round may be firable, just no explosive in the grenade. An associate of mine, asuming a blue device was inert, set it on the shelf above his desk, where it sat for several years, until it fell of the shelf, and exploded. A very small 'marker' charge went off, leaving bits of shrapnel around the room, but, thankfully, no major damage or injuries (except to the pride of the EOD guy who should have known better)
Merrill