It depends on what you mean by blob-free. There is no cellular modem on the market that doesn’t require proprietary firmware to function. The question is whether that proprietary firmware needs to be stored in the main filesystem of the device and then passed to the cellular modem when a device is booting or reinitializing the cellular modem. In addition, some cellular modems require that proprietary drivers be executed in order to interact with the cellular modem.
The Thales Cinterion (formerly Gemalto) PLS8 and BroadMobi BM818 modems both use free/open source USB drivers and they don’t require proprietary firmware be stored in the main Linux file system (at a location like /lib/firmware), however, both of them use proprietary firmware which is stored on their M.2 cards. Purism hasn’t yet provided instructions on how to update that firmware as far as I know. For the differences between the modems, see: https://source.puri.sm/Librem5/community-wiki/-/wikis/Frequently-Asked-Questions#218-how-are-the-thales-cinteron-pls8-and-broadmobi-bm818-modems-different
As for the USB version of the Thales Cinterion MV31-W modem, it uses the standard free/open source USB driver found in Linux 4.19 with a patch. See the diff here: https://iot-developer.thalesgroup.com/tutorial/modify-and-setup-linux-driver-mv31-usb-variant
I couldn’t find any info on where the proprietary firmware is stored (you need to sign up to be a Thales developer to get access). This modem is going to cost a pretty penny. Codico is currently charging 360 euros for it.
I didn’t think that it would be possible to fit a 5G modem on a 3042 M.2 card, because current 5G modems consume 2.0 - 2.5 times the energy of LTE modems, and thus require a lot more cooling than a standard heat spreader on a 30x42 mm card can provide. The MV31-W Hardware Interface Description (p. 42-44)) says that it should be attached to a copper plate and heat pipe on the top for full 5G functionality, and it has a gold-plated ground plane, which I suspect was necessary to conduct away heat. Gold is the best heat conductor available, but very expensive. Even if just being used for 3G or 4G, it recommends attaching it to a heat pad on the bottom (presumably with thermal paste), which won’t be possible on the Librem 5.
The MV31-W has 4 antenna connectors, whereas the Librem 5 only provides 2 cellular antennas, but from the manual it looks like only 2 are necessary for LTE and sub-6GHz 5G. Thales says that it will be able to support mmWave 5G with future firmware upgrades, but that requires 3 additional antennas. It supports up to USB 3.1 Gen 2 (10Gbit/s), but can use USB 2.0, and the Librem 5 only has a USB 2.0 bus to the cellular modem.
Maybe the MV31-W could be made to work as a global LTE modem in the L5 without any cooling, which would be an advantage over the regional PLS8 modems, but it costs a lot and the L5 currently uses both USB 2.0 and I2S buses to control the BM818 modem, whereas the MV31-W manuals that are publicly available don’t mention I2S, so I assume it will take some work on the software side to get it working in the L5 for LTE. 5G appears to be a non-starter, considering the cooling requirements.
By the way, it really cool that Thales posts its manuals online so anyone can download them without registering, and it has a public forum where anyone can ask questions. It is most open of the modem manufacturers, but its modems are very expensive (at least for ordinary people who just want to buy 1).
… and the potential exists that the modem can update its own firmware (FOTA) whether you like it or not.