VaJim reported this to Bloom Sky some time ago, others too.
The problem is that so far, the Bloom Sky reports an "absolute pressure" based on their installed sensor and whatever in house calibration they do (if any).
So far, they do not provide a way to correct the barometer reading to match an elevation corrected value, the barometer altimeter reading that we all mostly use (as reported by a local airport and news agencies).
I have also suggested that Bloom Sky give us a way to adjust the numbers (calibrate) that they send to WU. Or, that WU give us a way to calibrate sensor readings.
Of course a third option is that Bloom Sky gives us a way to program our individual stations (Bloom Sky firmware) with our own calibration constants.
There was a brief period as nincehelser noted some time back where there was difference between the Bloom Sky (different Bloom Sky pages) reported barometer values. The difference for my station was very near the correct difference for my elevation. So, I wonder if they were trying to do the elevation correction based on topo map data. The problem is that my unit's absolute calibration also needs a calibrated offset (it appears to report the wrong numbers, even just for uncorrected absolute.)
It is too bad that Bloom Sky has not yet addressed this issue. First, it hurts their credibility as providing a serious full wx station (as opposed to just a good webcam sky picture, which is nice). Second, there is a risk they could corrupt amateur crowd sourced wx data (although most are so far off without calibration, perhaps the barometer data is automatically discarded).