If you are using CQL, it is best practice to create your column families with CQL too. It seems you've found a bug, since the resulting CQL schema is invalid (there are two columns called key):
CREATE TABLE newdata (
key blob PRIMARY KEY,
key text
);
However, if you want just one field that is indexed, you probably want the following schema:
CREATE TABLE newdata (
key uuid PRIMARY KEY
);
Here, the primary key is of type UUID so a check is done that your key is a valid UUID. You can then insert with:
INSERT INTO newdata (key) VALUES (8768c481-a118-48b7-aed2-2903b917d045);
Then you can do lookups like:
SELECT * FROM newdata1 where key = 8768c481-a118-48b7-aed2-2903b917d045;
which will tell you if the UUID exists or not. You can add other columns later and retrieve them with the above SELECT query.