Why did he say that the blood should return on the head of Yo'av and his seed?
Radak: This is like David cursed him - "v'Al Yikares mi'Beis Yo'av?" (Shmuel II, 3:29).
Malbim: He explained why Yo'av deserves both death and David's curse. (a) He killed two men, (b) who were better than himself, (c) he killed them with the sword (which made it seem that David authorized him, for the king kills via the sword), and David did not know (this lessened David's honor), and (d) the victims were the head generals of Yisrael and Yehudah. Therefore, the curses will be on Yo'av and his seed, and not on David's seed. However, Chazal expounded that he 1 was not justified, and the curses were fulfilled on David's house.
Refer to 2:30:1:1. (Malbim did not clarify if he means that David was not justified [to curse him, since he intended also to kill him - refer to Shmuel II, 3:29:4:1*], or if Shlomo's reasoning was wrong. - PF)
