Since you are already using Libreboot, you already have (proprietary) microcode updates installed. So I think it shouldn’t be a security disaster with Linux-libre (that assumes that you keep your Libreboot updated). Worst thing that would happen is that your hardware won’t work. That’s also the best thing that will happen. The blobs are just firmware that gets loaded on a device that needs it. If you have the device, it won’t work without blobs. If you don’t have it, the firmware is not loaded so the outcome is not that different from regular linux. And also reading from comments there are some blobs for enabling DRM content. I guess that’s not mandatory.
Though imo Linux-libre is pointless. For noobs it’s a potential security disaster and skilled users would be better off compiling their own kernel with just the features they need to reduce attack surface.
Ok but Linux-libre does not solve the security risk. It just makes hardware not work. You might as well say that any kernel module is a security risk (be it Free or proprietary) and it’s better to turn it off.
Also unlike the blobs which “can cause risks”, Linux-libre causes risks. It removes proprietary microcode updates. So the outdated (also proprietary) microcode installed on your computer leaves you vulnerable to things like Spectre.
This is potentially not an issue if OP uses ARM for example but using Linux-libre for security reasons is a really bad joke.