Half-blood elf is the term used to describe the offspring of humans and blood elves. They do not yet exist or at least no in large enough numbers or ages to be noticeable(if any they are most likely infants). Since blood elves came into existence only a few years ago, they certainly have not had time to mingle with humans and produce adult offspring — without even factoring in blood elf prejudice. This does not mean that it cannot happen, however; it is possible it might happen 20 years in the future. The blood elven half is more dominate and they have the blood elven racial traits.
Like their blood elven parents, half-blood elves are addicted to the use of arcane magic. A half-blood elf must spend one hour each morning in meditation, resisting the distractions of their addiction, or have problems with spellcasting or spell defense for that day. Half-blood elves slake their thirst through the absorption of fel energies. If the blood elf partakes of demon's blood, the magic addiction abates for a number of days. Half-blood elves reek of fel power, offending the spiritual senses of night elves and tauren.
Most half-blood elves live in Dalaran and a few in Shattrath City because in Dalaran humans and blood elves live side by side. They are even more rare in Shattrath because The Scryers would not mate with the humans of Lower City. Some half-blood elves are born after the events in Schnottz's Landing.