Can an RFID-blocking card let me keep my building access card in my wallet?

A blocker only works if it substantially shields the tag or cards you want to block. A single blocking card between the access card and the rest of the wallet usually won’t do that, because radio fields can reach the cards from the sides and back. For reliable shielding, the unwanted cards need to be enclosed in conductive material, not merely separated by one layer. A simple test is to put several layers of aluminum foil around the other cards and see whether the access card works by itself.

The reader is probably getting responses from several cards at once, or the cards are interfering with one another. That can make the reader receive unusable data. Keeping the access card on the outside of the wallet may help, but the most dependable solution is to carry it separately in a badge holder or sleeve. You could also ask the building administrator what card technology the system uses, since some access cards operate differently from ordinary contactless payment cards.

Most RFID-blocking products are just conductive shielding, similar in principle to a small Faraday enclosure. Aluminum foil can work as a quick experiment, but the shield needs to surround the card on all sides for best results. A metal sheet behind the access card may reduce coupling or even change how the antenna behaves without fully blocking the other cards, so it may be unreliable.

The security aspect depends on the type of card. Modern access cards and contactless payment cards often use authentication rather than simply broadcasting a readable number, so blocking them is generally about preventing interference or unwanted reads, not protecting a secret that someone can easily copy. In your case, the practical issue is card-reader behavior, and physically separating the access card is likely to work better than relying on a blocking card.