It's garbled terminology, which is a problem by no means reserved for battletech.
In the original MW1; the Blazer was referred to as the Blazer "heavy carbine" and the carbine part has sort of suck with it and was reinforced through several notable books; the Twilight of the Clans Novel is one, one of the Warrior trilogy is another. But a carbine is a cut-down rifle; whereas the Blazer is two rifles stuck together. That's the opposite of cut-down.
There is no real carbine version of the Blazer; it's the same double-laserrifle it's always been.
There is a Blazer pistol; just two Laser pistols bolted together, but no carbine.
There is sort-of a heavier version of the Blazer though; The Hellbore Assault Laser uses two laser tubes which it fires in sequence to avoid over-heating one or the other.