Although the same verb is used, the two sentences do not convey the same thing. The first sentence translates to “there’s A blister that I saw on my chest” and the second, “I saw THE blister on my chest”. Therefore, in the first, seeing it was accidental, a discovery. In the second, it was intentional, i.e., you were looking for it and found it. I would agree with your use of “blister” because, as you described it, the “butlig” contains water/liquid. Normally though, the meaning of just “butlig” would be closer to that of a tiny wart or a dry pimple. The Tagalog word for “blister”, when it results from injury, e.g., a burn, abrasion, is “paltos”. Also, we would most likely use "pasasalamat" (gratitude) instead of "pagsasalamat" (the act or manner of saying "thank you").

I'm a native speaker and I would use the first sentence rather than the second. The difference is that the first can be literally translated as "I have seen a blister (with pus) in my chest." while the second can be translated as "I saw the blister (with pus) in my chest." Typically the first expresses either a possibility of seeing (may nakita means "to have seen") while the other usually is used to state a fact that "I have seen." Also the first usually is used to express "I have spotted" while the second is used to say "I saw." Hope this helps. PS pagsasalamat is by convention shortened to pasasalamat.

the first sentence translates as... I have seen a blister on my chest. the second one translates as, I have seen the blister on my chest.

yes,butlig means small abcess in your chest either a collection of pus,blood or water,