How to replace words in file in bash

I have a file with following content:

Blekota blaboli o koblihach.
Blanka je bl...
GEwI
er

I need to replace every word starting with Bl or bl with xxxx and save it into new file. I try this, but it did not work.

while read line; 
do pokus="${line//[Bl|bl].* /xxxx}" 
echo $pokus
done < "$TEXT" > "$TEXT".new

Desired output is:

xxxx xxxx o koblihach. 
xxxx je xxxx...
GEwI
er

What do I do wrong, please?

>Solution :

This can be done using a sed command:

sed 's/\<[Bb]l[[:alpha:]]*/xxxx/g' file

xxxx xxxx o koblihach.
xxxx je xxxx...
GEwI
er

Here \<[Bb]l[[:alpha:]]* matches a word starting with Bl or bl followed by 0 or more alphabets.