BOTH are to be blamed!
Adam and Eve were gifted with FREEWILL(the freedom to make there own choices and decisions).
Though, Eve was first lured into eating from the forbidden tree, two options were presented before her, either to accept or reject and this where FREEWILL comes in, but she chose to accept the offer among all other trees they had the freedom to eat from without restriction.
In turn, Adam ate the fruit Eve offered to him.
However, whether to accept or reject lies on him too. Sadly, he was deceived too!