People's statements on this type of subject often reflect the very narrow range of music they are familiar with. This is inevtable, who has the time to listen, and think seriously, about all the music in the world? So we often hear things that seem superficially sensible at the time, but become less so later. For example, a statement such as "the blues scale is pentataonic" is obviously untrue. Loads of blues don't use pentatonic scales. Some do. Much like Scottish folk music.Or English folk music.
There is also nothing specifically bluesy, or African, about varying the third or seventh in a scale within one song.This is a widespread practise in many cultures' music. Tunes with variable thirds don't have to sound bluesy. Likewise blusey tunes don't have to have variable thirds. As a matter of fact, I would say most traditional English singers used variable thirds and sevenths. They just did it in a different way from blues singers. A L Loyd discusses this at some length in "Folk Song in England".
Any statements about flattened fifths also need to be viewed with suspicion. Really noticeably flattened fifths are extremely uncommon in early blues(though they became more trendy as jazz modernised).
As with most folk music topics, there is more to this than meets the ear!