First off, while running a distribution free of binary blobs makes some sense in terms of human values and ideology, you have to bear in mind there are only limited sets of hardware that can be fully exploited by a Free GNU/Linux distribution. So it's not an easy road, quite the contrary. It all depends what you cherish most: features or independence.
Is LMDE fully made out of free software?
Taken as is: yes, it is made of free software. But not necessarily. There's a trade-off between «Freedom» and «Comfort» (i.e. «Features»).
Let's see it under another angle. There is a fairly high probability some of your hardware cannot run without at least one binary blob. Among the common hardware that I know of are Broadcom chipsets, nVidia, various bluetooth chipsets, some Ethernet and wireless network cards... Some of these have happened to, some still do require non-free binaries.
Fact, depending on your hardware you might be required one day to install a non-free binary if you want to fully profit from it. For some, ideologically speaking, this is unacceptable. On the other hand there are many people who are perfectly fine with that trade-off.
This doesn't mean of course hardware and software freedom shall be given up.
Back to your question: Debian allows for stripping out non-free binaries. Whether these being accessible from Debian repositories make it a Free distribution or not... is up to you. You will have at least to check LMDE repositories for any of Debian
nonfree to see if Free is applicable.
What I know is that Debian has a social contract and the people behind are doing their best to uphold it.