Yes, a masterwork blue ice longsword is mostly equivalent to a +1 longsword.
As you said, blue ice slashing weapons give +1 damage, and masterwork weapons give +1 attack. A +1 magic weapon gives +1 damage and +1 attack, so a masterwork blue ice weapon gives the same attack & damage bonuses as a +1 magic weapon of the same type (assuming it's a slashing weapon).
It's still not a magic weapon, though, so it's missing the other effects of magic on weapons. Most of those you wouldn't care about (the glow) or are actually good (not subject to Dispel Magic or antimagic fields), but it does mean your longsword won't bypass DR/magic damage reduction.
No, the bonuses do not stack.
From the PHB, p171:
Bonus Types: ... The important aspect of bonus types is that two bonuses of the same type don't generally stack. With the exception of dodge bonuses, most circumstance bonuses, and racial bonuses, only the better bonus works. ...
From the DMG, p221:
Weapons: ... Magic weapons have enhancement bonuses ranging from +1 to +5. They apply these bonuses to both attack and damage rolls when used in combat. All magic weapons are also masterwork weapons, but their master bonus on attack rolls does not stack with their enhancement bonus on attack rolls. ...
Magic weapons give enhancement bonuses to damage. Slashing blue ice weapons give enhancement bonuses to damage. Enhancement bonuses do not stack. Therefore a magic blue ice slashing weapon would only receive the higher of the two enhancement bonuses to damage.