Is this not rude:

I checked the code and I’m appalled. There are more BLOBs than source code

No. The commenter is voicing their own feelings and explains why they have them. There is neither blaming nor rudeness here.

And this:

I understand that removing BLOBs isn’t a priority over new and shiny features. But due to recent events, this should be rethought.

It would have been nice if you had explained why you think this is rude. The author expresses understanding that the maintainers’ priorities don’t align with the author’s. This seems to be an uncontroversial statement to me.

Then the author explains (I agree, it’s more a hint than an explanation) why they think the priorities should be changed. In my view their argument is sound. Again, there is no blaming or rudeness here.

They should have opened with a complement

I assume you mean “compliment”.

I’ve often heard of the “sandwich technique” – start with a compliment, then voice criticism, end with another positive thing. I find this is an appropriate procedure when voicing open feedback, that is, good things and bad things. However, this is a Github issue. Its whole point is to point out a perceived problem, not to give the maintainers a pat on the back or thank them.