What blinds Roderigo to fact that Iago is using him in "Othello"?
It is Roderigo's own desires that blind him. Iago persuades him by convincing Roderigo that Desdemona can truly be his. He sets up Roderigo as the "rightful" suitor of Desdemona, and Othello as the interloper. Because Roderigo so wants Desdemona, he easily believes Iago's lies and goes along with Iago's "plan", believing that he will be rewarded with his love.
Here is the speech:
It cannot be that Desdemona should long continue her love to the Moor—put money in thy purse—nor he his to her. It was a violent commencement, and thou shalt see an answer-
able sequestration; put but money in thy purse. These Moors are changeable in their wills:—fill thy purse with money. The food that to him now is as luscious as locusts, shall be to him shortly as acerb as the coloquintida. She must change for youth; when she is sated with his body, she will find the error of her choice. She must have change, she must; therefore put money in thy purse. If thou wilt needs damn thyself, do it a more delicate way than drowning. Make all the money thou canst. If sanctimony and a frail vow betwixt an erring barbarian and a supersub-
tle Venetian be not too hard for my wits and all the tribe of hell, thou shalt enjoy her; therefore make money. A pox of drowning thyself! It is clean out of the way. Seek thou rather to be hanged in compassing thy joy than to be drowned and go without her.
Iago tells Rogerigo that Othello, being a Moor, will quickly abandon his wife. He tells Roderigo that Desdemona doesn't really love Othello and will give him up for "youth". He suggests that Othello is a barbarian, and so the vows are "frail". Iago is playing upon the racism of the time, suggesting that Othello is less than a man because he is black. Roderigo wants to believe, and so easily does. Iago cleverly confirms what Roderigo is already thinking.
What is clever is that Iago intermixes the suggestion that Roderigo get rich with the idea that Roderigo is destined to be with Desdemona. Roderigo should raise money to lure Desdemona, Iago suggests. Really, though, Iago is going to be the one to benefit from the riches.