$1 Champlain Monument Stamp Question (Scott #227 - Sg # 351)

I have a plate block of 6 of the 1935 $1 Champlain Monument stamp (Scott #227 - SG # 351).

My father had noted down that this block is of the "light blue" variety which is very rare and worth more as a result.

I have searched through both Scott and Stanley Gibbons and have not found any mention of a rare "light blue" variety on these.

Is anyone familiar with a rare light blue variety of this stamp?

Below is a scan of the block.

Cheers... Marc...