Now kbrede,

If you were to pull the negative a little bit in the carrier so it shows a little air - you should get maximum black there - and it should be "close" to the maximum black of your film. This may rule out any impact "developing the film" may have confused you with.

(35mm has a higher density on purpose - but 120 usually is close to clear - so if you can get black on clear air you can get black anywhere).