Do blessings and curses have any effect on the dead?
We say things like, " may his memory be a blessing," "may the Neshamah have an 'Aliyah," etc.
What effect does it have?
Blessings have an effect. For example Kaddish takes the Neshama out of Gehenam and it raises Neshamot in Gan Eden to higher places (Shaar HaGilgulim Hk. 14).
HaRav Musafi writes in his Sefer (Shivat Sion) that when someone says Hashkava for someone after the first year of their death it can Has WeShalom have a negative effect and may Has Weshalom lead to the Neshama to get tortured. He also says that when someone reads the Haftara "Lezecher Nishmat" someone and makes mistakes it can also negatively effect the Neshama Has Weshalom.
So the answer is, yes our words and actions have effect up there.