What is the most hair friendly injectable steroid?

[Bonaparte]

Primo is a DHT derivative, there is no such thing a boldenone derivative. They are either Test, DHT or Nandrolone based derivatives. Also, DHT is the main the main culprit in hair loss and what the OP should be avoiding for his stated concerns.

It is 5a-reduced Boldenone with a 1-methyl group. Take away the 1-methyl group and you're left with what? Dihydroboldenone (1-Test), not dihydrotestosterone. And if this DHB were not 5a-reduced, it would just be boldenone, which is roughly half as androgenic as test.
So we're arguing semantics here and classification methods here, but anything with a 1-2 double bond (boldenone and its derivatives) is much less androgenic than its counterpart without that 1,2 double bond. (you're just looking at it broader than I, since Boldenone and its derivatives have a critical structural alteration from Test which makes them less androgenic, even though they are derived from test. I guess you can look at them as a hormone sub-category).

Primo is 5a-reduced, but it acts as a VERY mild androgen and doesn't really cause any of the normal androgenic side effects (acne, aggression, increase in sweating, increased sex drive, etc). So the fact that is is 5a-reduced just keeps it from being converted to anything else (like estrogen or actual DHT), but it really doesn't act like most 5a-reduced compounds (DHT derivatives, if you want). Also, Anavar is a DHT derivative, and that's as mildly androgenic as it gets.