Jealous. He said he was jealous. The concept was ridiculous to Blaine. If Finn had known what Blaine's life was like outside of Kurt and Glee club, he never would've said that.
The concept wasn't just ridiculous. It was foreign and alien. Blaine was jealous of Finn. Finn didn't have a pit in his stomach at the end of every school day because he ha to go home. Even though his real father had died, Finn still had a close relationship with his stepfather, Burt. Finn didn't have a broken family and a broken home.
Blaine got a pit in his stomach at the end of every day. Blaine didn't have any sort of father figure to look up to (even though he knew that if he really thought he needed one, he could look to Burt; both the man himself and Kurt had told him that). Blaine came from a broken family and a broken home. Only Kurt knew. But even so, Kurt didn't know the full extent of the damage Blaine's father does to Blaine almost every single day. Mentally. Emotionally. Physically...
When he was letting everything out at the punching bag, yeah, he was picturing Finn and Sam. The main anger and frustration he felt at the moment was towards them. But always deeper was the anger and pain he felt because of his father.
Yeah, he was picturing Finn and Sam. But he was picturing his father, too.
Finn had explained why he was jealous and Blaine had understood. But underneath it all was still the feeling that Finn didn't get it. Because he didn't. Kurt barely did. 'What's the point in trying to explain it?' he thought. 'No one would understand, anyway.'
But when he went home, he couldn't release his anger on a punching bag. Normally, he would try and just push it down, but it was too strong, too close to the surface. So he did the only thing he could do. He wrote a song and called it Jealousy.