Just went through the lessons - couldn’t find what lowercase i is - I know ‘I’ is the home chord - but for example: http://www.hooktheory.com/theorytab/view/james-blake/retrograde contains i instead. What does that mean?
it means a minor home chord.
The difference between major and minor is the third of a chord (chords are by default 1-3-5). If major is normal, minor has the third down by a half step (so 1-b3-5).
The tab you’re referring to specifically has a very complex chord structure, and those minor i chords are most likely borrowed from a different key.
EDIT: now that I’ve looked at it, the tab is in minor. This means instead of I-ii-iii-IV-V-vi-viio, you have i-iio-III-iv-v-VI-VII. You take the major scale, and start on the 6th note (so on a C scale, its relative minor is A).
This tab has chords borrowed from a different mode (here it’s major), explaining the purple I in the first measure.