Thanks again for the input!
Here is a pic of the blown cap, and how it fits into the greater scheme of things. It's a PME217 on the HV side. These are still available, though I'm not sure if my local electronics store will stock them. I'm sure we can get something of the same specs (240V, 220nF). The electronics store I know best has this:
(For reference, it's a CPS224 Class X2 suppression cap, with one model rated for 275V, 220nF)
Any reason anyone can think of that I shouldn't use this as a replacement?
Of course part of me wonders whether I shouldn't just wire the input straight to the transformer, but I suspect my enlarger lamp wouldn't last all that long if I did.
I really appreciate all the generous advice from you guys!