If I am asked to sign something saying that I agree to follow the policies that are provided from a website link, does that mean I'm agreeing to whatever the state of that website is on the day I'm signing, or any and all future updates to that website? It doesn't specify either way: neither "as of the date of signing" or "any and all future updates" is written. I'm not sure if that type of "blank slate" agreement is legal. This is in USA.
does that mean I'm agreeing to whatever the state of that website is on the day I'm signing, or any and all future updates to that website?
That type of "blank slate" agreements is lawful and it means "the conditions of which you are made aware", hence the terms as of the date you entered the agreement.
To supersede that meaning, the agreement would need to be explicit about referring to "any and all futures updates to that website". Your willful agreement of unknown future updates is described in Restatement (Second) of Contracts at § 154(b) as
[the party being] aware, at the time the contract is made, that he has only limited knowledge with respect to the facts [...] but treats his limited knowledge as sufficient
You might want to save a copy of the terms if you are concerned that the counterparty might unilaterally change the terms and allege that your acceptance was in reference to those.