Sorry...I read wrongly on questions ...

But the 19 is the total black blocks not both the blocks...

Very simple.

Every 3 white must be flanked by 2 black blocks.


We can see that the pattern repeats. We can group every 3 white blocks with 1 black block.


The last black block is alone and does not have 3 white blocks after it. It is only use to flank the 3 white blocks before it.

So only 18 black blocks are grouped with white ones.

(19 - 1 = 18)

This means there are 18 groups.

So since there are 3 white blocks in each group,

Number of white blocks used

= 3 × 18
= 54

This is how it looks like :

(1)-----(2)------(3)-----(4)------(5)... ... ....(17)-----(18)----(19)
■□□□■□□□■□□□■□□□■......□□□■□□□■□□□■