probably the socks. What kind do you wear? Cotton is not so good as it absorbs sweat and becomes soggy and causes chafing a blisters. Buy some Coolmax socks, or any other non-cotton slipperyish socks.
BTW, what shoes are you wearing? And where exactly are the blisters? If they're on the sides, it probably means you need wider shoes (if new socks don't help). On the tips means you need a larger size, and on the top usually means either the shoe is too small and pinching right there, or too large and your feet are slipping too much.