A DM could choose rule otherwise based on the senses a particular form of blindsight uses, but by default the the answer is "it only blocks vision".
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The DM "should" do whatever they feel makes theense for the situation in their game.
Blindsight is a type of vision. There is no logical reason for it to magically see through things that would otherwise be opaque; and there is no rules text that points to it being able to do so. It is not x-ray vision --that is a separate thing that does exist in the DnD ruleset (see ring of x-ray vision in the dmg) and can see through opaque objects like bugs.
So yes a DM could houserule otherwise, but by default blindsight does not work on SoSI's swarm, or otherwise see through any type of heavy obscurement; other than heavy obscurement caused by lack of light.