Your cats roam freely outside, right?

You might want to look into a payment plan with the local vet. If there is a
shelter in your area, ask them who they use - vets who work with shelters are
often more willing to work out payment plans with people, too.

If it really is an abscess, more than likely it will not go away on its own
and it may become so infected that it gets into the bloodstream and kills
Blackie.
I've had an abscess - they're horribly painful. Please try to find a way to
take him to the vet.
Also keep in mind that in some areas, it is against the law (animal
neglect/cruelty) to NOT take him to the vet. If you can't afford to care for
him, might it not be kinder to look for another home for him? If you really
love him, you do your best by him - even if it hurts you.

Stuff to think about. Good luck and I hope everything works out.

Depending on how queazy you are you could pop it yourself. I had a cat with
one years ago and the VAT just popped in and squeezed out the pus, put some
anti septic on it and said it's best to leave the hole open. All was well.

Last week No.2 cat was under the weather for a couple of days and then I
noticed a large wet patch on one of his shoulders, sure enough there was a
burst abscess there (probably from when he got mullered by a new rescue cat
I am trying to introduce into the house) so I gave it a squeeze and dab with
warm water to flush out the pus. Then plopped a blob of Savlon on it and
it's healing nicely now and Worf is back to his playful self.