This is a good idea, I’ll look into adding this. Unfortunately this is not supported by the API, but it should be simple enough to maintain a list of blocked posts in Arctic.
[edit] I see this is actually a native feature on Lemmy v0.19.4. I’ll add support for this in the next release. I just integrated this feature in Swimmy, (Arctic’s Lemmy API wrapper), I just need to update the UI to support this.