The answer to you question is no, there is no hard evidence that blackheads are caused by heating skin in the sun. This does not mean that it isn't true for you. Some patients are like you and report worsening of acne
with sun exposure and others actually report improvement. We do have evidence that androgen hormones, cosmetics, and possibly stress can effect the development of acne. Black heads are also known as open comedones. They are caused by the blockage of pores by dead skin cells and keratin. Sebum (a natural skin oil) then builds up in the pore causing it to enlarge. The sebum production is largely governed by androgen hormones which is why acne begins to get worse near puberty. If the sebum is exposed to oxygen, it turns dark giving the blackhead the distinctive dirt like color. Finally, sebum is a great place for bacteria to grow making the pimples larger and more inflamed. I suggest bring this issue up with you primary care physician
, whomever you see for treatment of your acne. He or she will make sure you are on the right track with respect your particular symptoms. Good luck.