The ex-Manchester United man feels the winger can be a key figure and ensure a team containing Cristiano Ronaldo can go far in the tournament in Brazil.
Nani should be fresh for the finals, after making only seven Barclays Premier League starts last term, and he shone in the final warm-up game against Republic of Ireland.
"I have to go for Argentina as my favourites," Blackmore told ManUtd.com. "I am feeling that Lionel Messi is due a good World Cup. They're strong going forward, although I'm not sure how good their back four is.
"As outsiders, from the European teams, I'd go for Portugal. When you've got the best player in the world, you must have half a chance. They have got other good players and I expect Nani to play well.
"He is one player who has not had many games last season and struggled at times to convince the crowd. But, as I'm sure the fans will realise, if he has a good World Cup, they have to get behind him when he returns.
"Getting on his back doesn't help him. I've seen him in training and he is frightening. Ronaldo is one player you can give stick to and it winds him up and makes him work harder. I think Nani needs to know the fans and manager are behind him to perform as we all know he can do."