Joseph Bory Latour-Marliac of France stated the following in The Garden, "The New Hardy Water Lilies", in 1893:

". . . important task remains to be carried out, namely, the hybridizing of the Castalias (today subgenus Nymphaea) with plants of the Cyanea section (today subgenus Brachyceras) which includes a great number of superb blue-flowered Nymphaeas. This is a work of which is well calculated to stimulate the enthusiasm of hybridizers."

Latour-Marliac stated again the following in The Garden, "Hardy Hybrid Water Lilies", in 1899:

"I should not bring this dissertation on Water Lilies to an end without bestowing a few words on the splendid section of the Cyanea, or blue Water Lilies. It is greatly to be regretted that hitherto all attempts to cross them with their hardy congeners of the northern hemisphere have so far failed. It would be a great triumph to add to the already sumptuous collection some hardy hybrids of a sky-blue colour with a delightful perfume."  

More recently Jacqueline Reditt wrote the following in the Cyprus Weekly, Christmas Edition 1989, "The Quest for the Hardy Blue", about Andreas Protopapas:

"You can never tell by appearances. Andreas Protopapas is, at face value, a mild mannered, conventional man who earns his living as the chief engineer of an offshore company based in Nicosia. Yet, after his family, his abiding passion is water lilies. And while other men dream of making their first (or next) million, a new car or scoring the winning goal for Cyprus in the World Cup, Protopapas devotes himself to creating the world's first 'hardy blue'.  

". . . A dreamer he may be, but Andreas Protopapas has few illusions about his chances of creating the elusive blue. 'It would be worth thousands of pounds but, to be honest, the chances are remote.' But he does have hopes for succeeding in the creation of a new hybrid in the near future, and he already has a name for it. 'It will be called Nicoletta, after my wife.' " 

Helen Nash wrote the following in Water Gardening (April 1996), "Perry D. Slocum: An American Legacy":

"Perry's hybridizing efforts have been primarily focused on two goals: developing free-flowering hardy water lilies that are resistant to crown rot and striving for the elusive blue hardy water lily. ……… The other of Perry's hybridizing goals, the blue hardy lily, is an exciting quest. At this time blue water lilies occur only among the tropical water lilies. The special challenge of trying to hybridize a hardy blue lily is overcoming the difference in chromosome count between the hardy and tropical species." 

Lastly, in the year 2007, an article "A Hero of Water Gardening: Joseph Bory Latour-Marliac, The Lily Man" on the website www.watergardenermagazine.com said the following about Latour-Marliac:

"Many of his experiments were doomed to failure because there was no possibility on a genetic level of there being any union between any of the hardy lilies and the tropicals. If he ever realized this, he never let on, but it did mean that his very ultimate ambition, the production of a hardy blue lily, would be forever unachievable, since the blue producing genes lie only with the tropicals of the Apocarpiae Group like the Egyptian Nymphaea caerulea."

Even though years and decades have passed, water garden hobbyists and enthusiasts still carry on the quest of their predecessors until the ultimate goal is met.

References:

Conard, Henry S. 1905. THE WATERLILIES, A MONOGRAPH OF THE GENUS NYMPHAEA. The Carnegie Institute of Washington. Washington D.C. p.86-87

Knotts, Kit. 2003 HOW DO YOU TELL THE DIFFERENCE BETWEEN TROPICAL AND HARDY WATERLILIES?. www.victoria-adventure.org

Latour-Marliac, Joseph Bory. 1893. THE NEW HARDY WATER LILIES. The Garden, Dec. 23, 1893: 582-584.

Latour-Marliac, Joseph Bory. 1899. HARDY HYBRID WATER LILIES. The Garden, Mar. 18, 1899.

May, Peter John.. 2007. A HERO OF WATER GARDENING: JOSEPH BORY LATOUR-MARLIAC, THE LILY MAN. www.watergardenermagazine.com

Nash, Helen. 1996. PERRY D. SLOCUM: AN AMERICAN LEGACY. Water Gardening. April 1996.

Pagels, Walter. 2000. CHROMOSOME COUNTS OF WATERLILIES AND OTHER NYMPHAEACEAE. www.victoria-adventure.org

Reditt, Jacqueline. 1989. THE QUEST FOR THE HARDY BLUE. Cyprus Weekly. Christmas Edition 1989.

Slocum, Perry D. 2005. WATERLILIES AND LOTUSES: SPECIES CULTIVARS AND NEW HYBRIDS. Timber Press, Inc. Portland, Oregon, U.S.A. 260 p.