When i detect starch with iodine is it suppose to turn blue black,or purple black

It depends upon the strength of iodine solution .

If it is weak it tends tobe towards purple and if strong it will be blue black .

click on the link below to see the result of weak iodine solution =
http://www.microbehunter.com/wp/wp-conte…

and now strong solution =
http://botit.botany.wisc.edu/botany_130/…
http://botit.botany.wisc.edu/Resources/B…