I didn't have to do this in my nail art section for my NVQ, but I've just had a look through one of the textbooks I bought (Nail Artistry by Jacqui Jefford, Sue Marsh & Anne Swain) and there is a design and how-to of opalesccent blending.
It appears they have applied a dark base colour and used opalescent paints with a fan brush to create a beach like scene up the nail. Using the fan brush with sweeping strokes across the nail. Repeat using another colour slightly above the first colour, moving up to a third colour all the way to the top of the nail (if you wish). Picture a beach scene with blue water and sandy shores.
There is a technical tip stating the less paint you use the more opalescent the finish.
HTHs