The Blastforge produces a resource called Behemium, more commonly referred to as blue. If the blue produced by a Blastforge is not used by the end of the turn it was produced, it decays and is not stored.
Short of special units in the random set, the first Blastforge is typically followed up with either a Wall or a Steelsplitter. Because Blue is best for defensive units, and quite poor at making attackers, you typically want to make a Blastforge only when your opponent is threatening to make attackers of their own. If you are under no threat, a Conduit or Animus is usually better.
Because the best absorber in the set usually costs Blue, players will almost always buy at least one Blastforge.
Player 1's natural blastforge opening is relatively uncommon, because you usually don't need defense as early as turn 4. Player 1's 3rd Engi Blastforge, which delays the Blastforge for one turn in order to get a larger economy, is extremely common.
3rd Engi Blastforge
Player 2 has a very hard time going Blastforge first. Usually, this is alright, because Player 2 can get a Blastforge out easily after either a Natural Conduit or 3rd Engi Natural Conduit opening, and Conduits are not very committal. However, in cases where, because of the Random Set, you really want a Blastforge and no Conduit, here are two openings you can use.
Called the masterbot because master bot uses it a lot. Mostly used in Flame Animus sets.
P2 3rd Engi Blastforge
Floats four Gold two turns in a row, but can be good in Big Blue sets.