Yeah, on the small scale road atlases anything up to mean high water is shown as "blue". On 1:50,000 ("Landranger") maps and larger maps those areas would be shown either as brownish (mud) or yellow (sand) with the mean higher water mark shown with a thick line and mean low water with a thin line. Blue essentially means "this bit is always water" (ie. it's below mean low water).

Maps in other parts of the world will show such places as darkish blue to indicate they are tidal. I prefer the Ordnance Survey way (but I am British).