thanks for sharing your concern.
Any dark shadow seen on radio graph suggest some pathology underneath the tooth.This happens in most of the cases.
But, sometimes radio graphs are deceiving too.I mean due to some technical error also you can see a dark shadow.This is called ARTIFACT in radiology.
Thats why only radiological evidence cannot justify a disease,it should be well supported by clinical symptoms as well.
Regarding your second query, different x rays are meant for different areas.
it is taken at different angle too so that one can see, what he/she wants to focus on leaving besides other structures, even though they may or may not appear in the x ray.
for example PNS view,occipital view,TMJ
view,OPG,bite wing etc.
you must visit your oral surgeon with your x rays to rule out any pathology like periapical cysts,tumours,etc.
hope it helps.