select string that doesn't contain a substring/field

TableA.content is varchar2(4000), which was populated from a CLOB value that contains markup and text for a website, paragraphs actually.
TableB.keyword contains a string that I want to find in TableA.content.

I want to do something like this, but it is not quite right. What am I missing?

select tableA.content from TableA
where (select distinct(TableB.keyword) from TableB) not in TableA.content;

So I want all the rows in TableA that don’t have any of the values in TableB.keyword.

I was checking out MATCH or CONTAINS but I can’t get it right using those.