A quick search on Google Scholar revealed this paper. The abstract reads:
The life settings in which sudden death may occur can be classified into eight categories: 1 on the impact of the collapse or death of a close person; 2 during acute grief; 3 on threat of loss of a close person;  during mourning or on an anniversary;  on loss of status or self-esteem;  personal danger or threat of injury;  after the danger is over;  reunion, triumph, or happy ending. Common to all is that they involve events impossible for the victims to ignore and to which their response is overwhelming excitation or giving up, or both. It is proposed that this combination provokes neurovegetative responses, involving both the flight-fight and conservation-withdrawal systems, conducive to lethal cardiac events, particularly in individuals with preexisting cardiovascular disease; other modes of death, however, were also noted. Better understanding of the potentially lethal life situations and identification of individuals at risk may lead to the development of practical prophylactic measures.
Other sources claim that Voodoo Death is caused not by an actual curse, but by the fear of the victim of the curse. I don't have access to the paper, which is cited as
Voodoo and Sudden Death: The Effects of Expectations on Health Transcultural Psychiatry January 1, 1982 19:75-92
In conclusion, it appears that this phenomenon exists and is under scientific investigation.
As for the sudden whitening: There are some historic cases, but these most likely did not happen "overnight", but rather over weeks and months. In addition, hair did not suddenly turn white, but rather a mixture of pigmented and white hair was already present, and the pigmented hair fell out. A nice overview is given in this paper. A relevant passage:
While these theories hardly stand up to even indulgent criticism, there is one explanation which appears plausible. Alopecia areata is a disease in which either patchy or diffuse hair los occurs, often in a matter of days and sometimes to an alarming degree. The cause is unknown. Many times a psychic event has been linked to its onset, but whether this was a post or praeter hoc relation is disputed. For unknown reasons the hair loss in alopecia areata confines itself almost exclusively to pigmented hairs; white hairs remain. Moreover when hair regrows in areas of previous loss, it often does so without restoration of its previous color. It is reasonable that in a person who had a mixture of pigmented and white hair, in whom rapid, difuse pigmented hair loss occurred, would appear to have grown white overnight.
For some of the historical cases, there appears to be another, much more banal explanation, found in this more recent historical overview:
The use of temporary hair dyes prior to 1907 may explain some cases of sudden canities. When incarcerated and sentenced to death, it is unlikely that individuals would have access to hair dye or servants to aid application.