It is completely legal, and not all that uncommon--for example, when people relocate across country (or from another country), such as for job reasons, they often have to lease without physically viewing the premises.It may be a bad idea (at least when there is any other option), but it's not illegal. If a landlord refuses to allow you to see the inside of the premises, that almost certainly means he has something to hide and its an especially bad idea to do this...but the law does not make bad ideas illegal. If you choose  to sign the lease in that circumstance, it will be enforceable.