Asphalt and tar paper are often used for an absorber in this type of direct solar coiled tube water heaters in Mexico.
The downside of plastic tubes with hot water (or cold to a lesser degree) is leaching of plasticizers into the water; even more problematic than direct absorption from the skin contact is breathing it in with water vapor during the shower as well.
HDPE is likely the lesser of evils, but from what I've read it depends entirely on additives used; no telling what is in black irrigation tubing to resist UV so well. Testing of endocrine mimicking compounds in plastics is still only in the hands of a few private companies here in the US. Phalates are only one group, with many others having been identified as equally estrogenic, or having other endocrine impacts.
Maddening that virtually no useful information is available in order to make an informed plastic material selection choice. Likely just a result of incompetence but suspicious given how long this issue has been known, as well as the known 50% reduction in male sperm counts, and 1 of 5 breast, uterine, testicular, prostate cancer rates as well as increase in thyroid disease.