The problem is not the pitchblende itself.
The problem is the decay biproducts. For example, polonium, and radon gasses.
Dwarves are subterrenean. They don't get good ventilation. (Miasma anyone? Hehehe.) Radon is a clear, colorless, ordorless gas. It is released from uranium as the uranium decays. Even small traces of uranium will release measurable levels of radon. This is why radon is a serious problem in some central plains states, like my own.
The sand and gravel used to make concrete has harmless trace quantities of uranium ores in it, and slowly releases radon gas over time. (Far lower concentrations than a formation of pitchblende would.) The radon itself isn't terribly radioactive either, but decays fairly rapidly into polonium, which is highly carcinogenic. This manifests as a fine white powder of polonium oxide which is normally invisisble and or indistinguishable from ordinary dust.
Inhalation of radon has the risk of spontaneous decay of the radon in the lungs, turning into polonium which oxidizes and deposits in the lungs, and accumulates like black lung. Over time, it causes serious health issues.
Take for instance, the polonium present in tobbacco. When you smoke the tobbacco, the polonium sticks in your lungs, irradiating them. Smoking a pack of cigs is equivalent to several chest xrays. This is one of the many reasons smoking causes cancer.
Sleeping in that artifact pitchblende bed, in a poorly ventilated, enclosed space for prolonged periods of time will have measurable health risks.