1.苍 is not a usual color word. It is often used in idioms or literary texts when it imply colors.
I don't read many books,I only know two phrases using 苍 for color:
e.g. 苍天啊,大地啊,是哪位天使大姐替我出的这口气啊. from a comedy dialog
2).皓首匹夫,苍髯老贼....from "Romance of the Three Kingdoms"
Both 皓 苍 mean pale here.
青 has two meanings,in nowaday life,青 is cyan.It is not blue or black ,it is cyan,sometimes 青 is greener than cyan.Just google cyan,and you will know it,but don't google 青,those 青 pictures should be 蓝. You can baidu 青 , the results is almost identical to google's.
In ancient or old days ,青 may refer to dark blue mixing dark green. And the result for those color-insensitive people seems like black.
So when you see 青鸟,青草,青whatever, It maybe cyan,dark green,dark blue, But it won't be black.Because 青 has nothing to do with black.