Is it the name of the insect or the result of it producing a scale on the needles?
Both
Insects and diseases are often named for what they appear to cause or how they appear to the individual observing them (oak wilt, red oak borer, redheaded sawfly, etc.). In the case of black pineleaf scale you would see somewhat hardened black bumps on the pine needle (thus you get "black pine leaf scale").
The "scale insects" are a group within the class Insecta that feed on plant sap (there could be scales that feed on other things, I just don't know of any).
Synonyms of scientific names are important - but only a little in practical application. It helps you if you are looking at older literature or discussing the issue with someone who has been around for a while. Basically, scientists change the name for one reason or another.
For example. Hypoxylon canker of aspen caused by a fungus named Hypoxylon mammatum has more recently been described as Entoleuca mammata (there are also other synonyms for this fungus). Depending on who you were talking to they may only recognize it as Hypoxylon mammatum and if you only knew it as Entoleuca mammata it would be beneficial to know the synonyms to realize that you both were talking about the same organism...generally discussions like this will only occur in the scientific community, all others would just refer to it as Hypoxylon canker and all would be fine...very confusing
