Do you want to search the list of filenames that the ls command returns, or the contents of the files themselves?
To list the contents of your ~/downloads directory and look for occurances of the string "bleh" in the filenames you could do something like this: ls ~/downloads | grep "bleh"
To go through the files in a directory and display what files contain the string "bleh", and in what file and on what line, you could type this: grep -Hn bleh *
The -H switch displays the filename, the -n switch displays line number in the file(s) where "bleh" is found, "bleh" is the string to search for and * is the mask for the files to be searched (an asterisk meaning all files).
To recursively look for files containing the string "bleh" in their name, try this: find . -name "*bleh*"