The problem is, DRM is fundamentally incompatible with open source.
If you have an open source DRM mechanism, what stops anyone from just looking at the source to figure out how to bypass the DRM?
So if GStreamer wants to support any DRM scheme, it would mean binary blobs for sure.
DRM goes against user freedom, it's a hostile technique used to take away the control of our hardware from us. No Sony rootkits for me thank you!
In other words, GStreamer's DRM support would find effective usage on user-restricted hardware such as phones and tablets but not on open source OSes where anyone can look at the output of any "binary blob", thereby defeating the purpose of the binary blob itself.
The only way an "open source" implementation of DRM could work would be if the actual DRM decoding would be done remotely, ie. the local open source player would send the encrypted data to a remote server, which would stream back the decrypted video. The remote server would act as the black box in this.
Of course, you don't have to be a genius to figure out a million flaws in this idea.
On the other hand, GStreamer developers could simply break the blu-ray encryption, and create an open source implementation that plays back DRM'd content without the DRM. That'd be fine. Commendable, even. Isn't DVD playback already pretty much done in this way anyway?
"It's always possible to use GPLed code to write software that implements DRM." -- A Quick Guide to GPLv3
Even that, I don't think GStreamer's solution for Blu-ray will be open source. It could be in a form very similar to their DVD playback solution: commercial closed-source product with required license and patent from Blu-ray group, based on GStreamer technology.
Still, I think it's good to have it on Linux, just as Steam.
It's a historical burden Desktop has to carry until we reached DRM-Free world.
GPLv3 is designed specifically to prevent using laws against users in the form of DRM. That means if you write a DRM scheme and license it under GPLv3, then by the GPLv3 license, you can't sue anyone else for bypassing this DRM scheme, because the GPLv3 license protects users against DRM.
Now with blu-ray this is entirely different, we're talking about an existing DRM scheme, and in order to write an open-source implementation that would allow the use of DRM-restricted content, there's only two possible solutions: one, GStreamer figures out and breaks the DRM restriction, and writes an implementation to exploit its weakness, or two, GStreamer gets specs from blu-ray and uses them to implement their DRM scheme. But since doing an open source implementation would effectively nullify the DRM restriction, number two is never going to happen.
So the only way it could work would be a closed-source binary blob.
It's possible to write a DRM in an open source way. From this aspect, not fundamentally incompatible. :p
I agree the rest of your saying, just as the 2nd part of my post saying: in the case of Blu-ray with an existing DRM, it could be another a closed-source implementation as their current DVD one.