Clarification...
IMHO, if our hypothetical blogger were writing about their own legitimate work in physics, then I personally would be fine calling that person a physicist, whether they were trained or not. (Others may disagree.)
There are, for instance, plenty of self-educated historians and mathematicians kicking around, and the lack of a shingle in no way detracts from their accomplishments...in fact, it's really rather impressive that they can maintain long-term focus without the dubious benefit of having some professional egghead breathing down their neck.