What is BLANCH in JOUNEN BLANCH. BLANCH & BLAN are the same, right?

Yes they are.
blanch (or blan) → white
yon fanm blanch → a white woman
yon mi blanch → a white wall
yon fèy papye blanch → a white piece of paper.

But blanch in "jounen blanch" means deprived or devoid of something
M pase yon jounen blanch.
I had a boring day.
I spent day devoid of food and fun.

same goes for "nuit blanch"
M pase yon nuit blanch.
I had a sleepless night.

and also "lari blanch"
Ledimanch, lari a sanse blanch nèt.
On Sundays, the streets are practically empty.

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words