Dungeon mob spawners do not take the blocks around it into account when spawning mobs.
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I've had the same issue also with a spider spawner even when lit up. Seed in attached pic which shows spider suffocating in wall to left of chest and spider jockey in floor next to spawner. Coords are X 165 Y 53 and Z 498. This is in 14w25b survival in newly generated world with no mods using WinXP and Java 1.7.0_60.
yeah I think that the inside of blocks are dark which would be why mobs spawn in well lit places on top if it not taking the actual blocks into account.
The range of the spawner seems to be the same, but it doesn't matter what's in the way or what light there is; mobs continue to spawn.