The rank colors are associated with a percentage of the total active player base. Around January I remember this was 3000, now is around 3800 (go to Community > Ladders > Show full ladders). This is influenced somehow by server restart (after a server restart number of active players goes down).
In conclusion, if we get more players => the number of blue players increases => the difference of best blue versus worst blue player increases.
For me the solution is to look at the numeric rank, not only at the color.
EDIT: and anyhow blue should be ~1% to ~5%. 4% difference in rank at that end seems large anyhow, so yes, I think there is a big gap. But this is not only for blue but similar for white (5% length) or yellow (10% length).
