Basically
blanch leeks are grown for their long blanched stems, which can be blanched to around two foot long.
There are various methods of obtaining long blanch length, the most common being drain pipes.
Whereas
pot leeks are grown more for the volume of edible parts, i.e. girth as opposed to length.
In fact as I recall with show 'pot leek' exhibits the blanch must not exceed 150mm (6") long and the larger the girth the better.
With blanch leeks both dimensions mus be as big as you can get them.
Generally pot leeks are more hardy and make good winter greens, the pot leeks below were picked this morning.
In the two pictures below ( which are not of 'show quality) the blanched leeks are about 600mm (2ft)long and the pot leeks have about 75mm (3") girth.
Blanch leeks
Pot Leeks;
If you want to see some really prize specimens have a look in the show vegetable section of my picture gallery.
I hope that answers your queries.