About 94.6% of caesium-137 decays by beta emission, so any Geiger counter would have shown abnormally high readings.

Would they not have been effectively blocked by the concrete wall the capsule was embedded in?

Depends on the position of the capsule in the wall. Also, this was in a 5-storey building, where load bearing walls are between 12 and 20cm.

If there was enough beta radiation to cause leukemia, there was enough for the Geiger counter.