Definition:
"Blue blood" is a term from medieval Europe. People believed that royalty and nobility had blue blood. The elite had enough power and wealth that they could afford to have peasants and the urban poor do their dirty work for them. Since the aristocrats were able to stay inside and avoid long hours in the fields (and the sunlight), they were often so pale that their blue veins showed under their translucent skin, thus leading people to believe that their blood was blue.

Additionally, the daily use of silver forks, spoons, knives, drinking goblets, plates, etc. by the Royal families and nobility caused the condition of Argyria, which literally turned skin blue and developed rather pronounced blue veins and arteries.
Scientists now know that the very high use of silver eating and drinking conveyances means that there was a very high transfer of ionic and colloidal silver mixed into the alkaline and acidic foods. Most directly the silver wine goblet could leach 100s of parts per million of silver in the wine.

Nobility:
Read more about Dutch nobility here. In short: Dutch nobility virtually expired after Charles V (1500-1558) became the ruler. However, if you can trace your family tree back to ~1500 or so, then chances are that your ancestors were of noble descent.

Search if you have blue blood (just for fun):
Do you have blue blood (a few drops maybe)? Want to check if you are a descendant of nobility? Try the following:

• All the van Voorthuizens (in 6 different spellings) in our database.
• Find your own name (or father's, or grandfather's) under the Search tab.
• Click on the name to open this type of screen.
• Then click on the "Relationship" tab under the person's name.
• On the new screen type 4251 in the second small white box, under Person 2 and then click on the Calculate button. Example here.
• If there is a hit, then you will see "Teuntje Gijsberts van Dorland" topping the result list. She is the wife of Hendrick Berends van Osenbruggen (~1640-~1695). The lineage to her ancestors goes through Gijsbrecht II van Nijenrode to the Brederodes, to Willem I, Count of Holland (1166-1222). Look here for a complex chart and from thereon to William the Conqueror and Charlemagne (Karel de Grote) -748-814. Look here.
  

Or try the above steps with the following numbers:

• 4242 =Maijken Dirckse Curff, the wife on Willem Hendrickse van Osnabrugge (1672-1745)
  Her chart also goes back to William the Conqueror. Look here. For a much more picturesque description and chart go here.
• 1 =Engelberta van Brienen (1653 - 1716). There are very many layers of nobility in the descendants of Engelberta van Brienen. Audrey Hepburn was just one.