Good faraday cage to block bluetooth beacons
As @mkeith says, any electrical conductor will block radio signals. Fine wire mesh does just as good of a job for high frequencies as tin foil.

However I would like to target something else in my answer. You say that you are receiving a signal strength of about -70dB - presumably that is -70dBm (the m is important). In that case the receive power is only 100pW (picowatt). At -90dBm it is only 1pW. You are probably being exposed to hundreds of times that power continuously from any Wi-Fi network or mobile phone in your house and likely your neighbours house too.

Furthermore, the beacons will not be transmitting continuously. They transmit for only a few milliseconds at some update interval (typically 1-10Hz), so the average power will be even less.

Basically what I'm trying to say is there is little point going to the effort of shielding them, there are so many other radio signals around that it will make no noticeable difference at all to the amount of energy in radio waves flying around in your house. Unless of course you are also going to wrap your entire house in tinfoil.