Bluetooth signal through survival blanket
If the foil blanket is far enough from the antenna (a few wavelengths) it is likely that the radio signal will be attenuated by 10dB to 30dB. A similar problem has been studied for window glass with thin metallic coatings, for example see figure 8 in https://www.diva-portal.org/smash/get/diva2:882367/FULLTEXT01.pdf

If the foil blanket is closer to the antenna than one wavelength there may be interference and the attenuation could be significantly worse than 30dB.

The metal coating on the plastic is very thin and the best scenario for sending radio through the blanket is likely to be if the foil blanket is one centimetre or less from the antenna. But this may not meet the thermal insulation requirements. 

Bluetooth operates at 2400MHz, the same as 802.11b/g/n, so a 2400MHz WiFi antenna will also work for Bluetooth and the antenna may be placed outside the foil blanket if you can purchase a HC-05 module with u.fl antenna connector.