Broadly speaking, your average rural blues musician had very little real chance of being recorded until the 1920s. Firstly, the advent of electrical recording in the mid-1920s radically changed the possibilities of recording. Before the mid-1920s, recordings had to be of something of a certain volume and only had a very limited frequency range. This led to the advent of instruments like the Strohviolin, essentially a violin with a trumpet horn placed over the soundhole, in order to improve a violinist’s chance of being heard on a recording. A blues musician, singing quietly in a corner, was not going to be picked up with enormous fidelity by the recording equipment that required Strohviolins.

Additionally, in the mid-1920s, the recording companies discovered that there was a market for what they then considered ‘ethnic music’ of all kinds, from ‘hillbilly’ music to the folk music of the Polish community in, say, Iowa, to southern African American blues. Before this they’d predominantly recorded a lot of classical music, big band dance music, operatic-style tenors singing sentimental music, and from 1917-ish this new kind of raucous dance music called jazz. All of which was loud enough to get recorded before electrical recording techniques with microphones.

In 1923-1924, you start to get these very successful recordings of urban blues singers like Ma Rainey and Bessie Smith, whose music became particularly popular. Such music, today, sounds more jazzy rather than bluesy, and indeed Ma Rainey had the likes of Louis Armstrong play trumpet on recordings. But this was very definitely the blues - it was just an urban blues, rather than a rural blues. It’s largely the rural blues that is prized today, almost a century later, but the urban blues was by and large more popular at the time. Anyway, it’s from about this point that the idea of a blues musician recording started to feel possible.

In early 1926, amongst the craze for ethnic music, Paramount Records took a Texas blues musician called Blind Lemon Jefferson to Chicago and recorded him singing a mix of blues and gospel tunes while playing solo guitar. Jefferson was very successful, and basically every record company wanted a rural blues musician of their own, singing over guitar, meaning that there was a place in the recording market for the likes of Blind Willie McTell, Mississippi John Hurt, Charley Patton, Lonnie Johnson etc. But by and large, with certain exceptions, this ethnic music that we’d now call rural blues was a niche music.

However, let me stop you there: so far, in this answer, I’ve been talking about blues musicians as if they were a normal thing that had previously existed outside of recordings. But there was not really any such thing as a blues musician until Ma Rainey or Blind Lemon Jefferson. These singers were, by and large, versatile singers who would play whatever music was popular, like working musicians anywhere. Their repertoires in their live shows were much more varied than just the blues. But the record companies generally wanted a) a specific ‘ethnic’ sound that had sold records, and b) new material. The blues happened to be a traditional form of sorts that southern African-American musicians like Bessie Smith or Charley Patton could ‘sell’ to the record companies, and a form that was durable enough to allow improvising new songs in the spur of the moment. So whatever your repertoire as an acoustic guitarist/singer in Mississippi, your best bet to get recorded was the blues.

So your rural blues musician wasn’t really a rural blues musician, most of the time, and didn’t get to record until the mid-1920s. And then, more or less, the Great Depression happened, and by 1930 many of the record companies of the mid-to-late 1920s - the ones who had recorded the blues musicians - had gone bankrupt. So interest in recording blues musicians - with some exceptions like the famous Robert Johnson or Leadbelly - basically died until after World War II.

In the case of the Mississippi Delta in the period from 1925 to 1940, in particular, the owner of a music store in Jackson, Mississippi, one H.C. Speir, was a sort of music scout. Speir had a fairly primitive recording setup (which Speir knew how to use as a former recording engineer) and he would record local musicians and send the results up north. And as a music store owner selling records, Speir had an idea of what kind of records sold, and understood the need for musical novelty and something...homespun; rural blues records largely sold to more urban African-Americans wanting some rural nostalgia.

So essentially, the process for a Son House or Skip James or Robert Johnson that led to their entering a recording studio was to travel from rural Mississippi to Jackson, and to try to convince Speir to record you and send that record to the Northern-based record companies.

However, in truth, such musicians probably largely saw a recording as a fairly pie-in-the-sky thing - a craze that made them a bit of money for a couple of years, while the recording companies were interested (typically the musicians would get a flat fee per song recorded). I’m not sure the blues musicians recorded in the 1920s really understood the power of recordings, apart from the select few that became particularly successful - Blind Lemon Jefferson or Bessie Smith. Most blues musicians would record a session of material and then disappear into the ether, only knowing if their recordings had made an impact if they were invited back for more sessions (by which time they could well be dead or have given up playing music, or have lost contact with the equivalent of the H.C. Speir who could tell them the record company wanted more material - perhaps the musician had moved to a different state altogether). There were no Billboard record sales charts, record companies were not inclined to share sales figures, and with exceptions like Speir, many of the places you could buy blues records were essentially the furniture stores who sold gramophones - the blues records were treated, basically, like furniture accessories. So often they were sold by people who didn’t know what they were, let alone who understood their appeal - it was just that sometimes African-American people had enough money to buy a gramophone that would play 78s, and the retailer wanted to be able to sell 78s they could play.

One of the things, it’s been argued, that sets Robert Johnson apart from those who recorded in the late 1920s was that he recorded around 1937 and that he sounds like the first blues musician who played like he’d learned things about the blues listening to records, who had an intuitive sense of their power. But basically many blues records that would be basically worth thousands today (at least) were recorded, sold and bought by people who weren’t that into the music. Famously, when Paramount Records went bankrupt in the Great Depression, they threw their remaining stock in a nearby river, not imagining that anybody would care about that music again.

And indeed it wasn’t until after World War II and the rise of record collector culture that a group of fans started identifying the late 1920s acoustic blues music as something special, and it wasn’t until the rise of the folk festival circuit of the late 1950s and early 1960s that names like Son House or Charley Patton started to be more widely known, something that then snowballed as blues rock groups like the Rolling Stones or Canned Heat sifted through that music for songs that would work in a more rock’n’roll context - all of which was probably quite bemusing to the blues musicians (like House) who’d survived into the 1960s and who discovered they were being feted by the cognoscenti, even as they’d put away the guitar and moved away from the Delta and got on with life and building families etc, and who often had to relearn their repertoire from the records they’d made long ago, and who probably would in some cases prefer to be playing gospel music rather than that old blues stuff.