Blister's Hideout was located in the Bay of a Thousand Scales, as mentioned in The Dark Secret and Assassin. This is where she resides with her soldiers. Her hideout's location is not shown on any map of Pyrrhia, from the first or second series.
She revealed that she had tricked her soldiers into thinking they were each exclusive elites who reported to her in secret, allowing her to know everything about her soldiers and the SeaWings. Apparently they would each report even slight acts or talks against Blister, which she would then swiftly kill to stop the 'traitor'.
After the war ended, the hideout was most likely abandoned, but could still be in use.
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