Manual linebreak with justification but without empty line afterwards

if the last line of your paragraph is really nearly full, you can get it to end flush right by the following approach.

• wrap the paragraph in braces.

• end the paragraph with these commands (inside the braces):

  \unskip\parfillskip 0pt \par
  

update: a mechanism that doesn't require braces.

• before the paragraph, insert

  \startsquarepar
  

• at the end of the paragraph, insert

  \stopsquarepar
  

• define these two commands in the preamble as follows:

  \newcommand{\startsquarepar}{%
      \par\begingroup \parfillskip 0pt \relax}
  \newcommand{\stopsquarepar}{%
      \par\endgroup}
  

it's not necessary to have blank lines either before or after either of these commands (the grouping and included \par ensures that that's not a problem), but readability and "good practice" suggests that a blank line before \startsquarepar and one after \stopsquarepar is a good idea.

blank lines within the scope of \startsquarepar and \stopsquarepar will start new paragraphs as expected, and all paragraphs within this scope will end flush right -- whether or not there's the right amount of material in each paragraph to avoid excessive spacing.

why didn't i use \begin... and \end...? while there are no "strings" attached to \begin..., latex has co-opted the use of \end... for its own purposes in defining environments. if you really prefer the \begin ... \end terminology and environment syntax, it's easy to transform the two command definitions given into a single environment definition. (but left as an exercise to the reader.)

edit: one final suggestion from egreg's comment (to keep it from getting lost).

{{\parfillskip0pt\par}} can be a useful for a manual solution as it doesn't need bracing the paragraph.

It is as simple as getting rid of the empty line in your code

\documentclass{article}
\begin{document}
Blah blah blah blah blah blah blah blah blah Blah blah blah blah blah blah blah  
blah  blah blah Blah blah blah blah blah blah blah blah. \linebreak
Blah blah blah blah blah blah blah blah blah Blah blah blah blah blah blah blah blah 
blah blah Blah blah blah blah.
\end{document}

Which produces

Empty line ends the paragraph, just like the \par command. \linebreak switches to next line within the same paragraph.