Most definitely he had a particular sound. He has 41 known recordings. He recorded 29 singles in two different recording sessions in TX in 1936 and 1937. He recorded 12 alternate versions. His uniqueness comes from his true bluesman story, combined with amazing guitar playing that mixed leads and chords. He was a roamer, a traveler, keeping a woman in most towns he visited - often picking the "marginal" or "unattractive" girls because they were more likely to be appreciative of the attention and would wait for his return.
The thing I remember most about him was reading somewhere that Eric Clapton said almost no one would ever attempt to record or repeat his songs. Particularly, "Hellhound on My Trail," because while it is written in the traditional 1-4-5 structure of the Blues, he (Johnson) had a way of transcending the the 1-4-5 structure, creating something completely unique, and then just when you thought it was too far gone to maintain the structure of the song, he would weave it perfectly back into the 1-4-5. A magical and almost indescribable application of the Blues.
As a note, Eric Clapton just returned to Texas with Doyle Bramhall II and recorded Robert Johnson's songs in the two hotels where Johnson recorded 80 years ago. They released it as "Sessions for Robert J." and it is well worth the price.