Both Black Mesa and Aperture Science are trying to develop portal technology, they're basically competitors. Aperture ended up falling apart (proly because of the death of its CEO, Cave Johnson), while Black Mesa's experiment caused an apocalyps instead, throwing the world into chaos and opening another dimension of aliens and headcrabs.
Aperture Science made better portals; the portal gun, yes, which is first invented on the Borealis(research ship). We all know that the proper name for portal gun is handheld portal device. "Handheld", which means there is something originally bigger, which is proly located at Borealis. BUT the ship disappears. Gordon finds the location of Borealis in Half-Life 2. Why? Because, Black Mesa decided to steal the ship instead.
Also, one of the prerecorded messages that Cave Johnson left was:
"Greetings friend. I'm Cave Johnson, CEO of Aperture Science. You might know us as a vital participant of the 1968 senate hearing on missing astronauts, and you've most likely used one of the many products we invented but other people have somehow managed to steal from us. Black Mesa can eat my bankrupt..."
You can clearly infer that Johnson blames Black Mesa for his company going bankrupt, and because of the "products we invented but other people(Black Mesa)... managed to steal".
10/10. Would make stupid conclusions again.