documentclass[11pt, parskip=false]{scrartcl}

% Bibliography preamble
usepackage[giveninits=true, style=philosophy-modern, yearleft=true, dateabbrev=false]{biblatex}
addbibresource{testbib.bib}

    DeclareFieldFormat[online]{title}{#1}

    renewcommand{postsep}{% Add comma to end of author section
    addcomma
    nullparnobreakvskippostnamesep%
      hskip-bibhangignorespaces}

    DeclareFieldFormat{urldate}{% Reformats urldate field to read "accessed", replacing "(visted on)"
        accessed %
        thefield{urlday}addspace
        mkbibmonth{thefield{urlmonth}}addspace%
        thefield{urlyear}isdot}

    renewbibmacro*{url+urldate}{% Makes URL begin on a new line, adds comma after URL, before URLdate
    printunit{newline}usebibmacro{url}%
    iffieldundef{urlyear}
        {}
        {setunit*{addcommaaddspace}%
        usebibmacro{urldate}}}

begin{document}
Sentence containing citation parencite{blogWithoutAuthor, blogWithAuthor}.

printbibliography
end{document}

@online{blogWithoutAuthor,
date = {2018},
title = {Historians in the News},
maintitle = {History Matters},
organization = {The University of Sydney},
url = {http://blogs.usyd.edu.au/historymatters/2018/02/historians_in_the_news_2018.html},
urldate = {2018-02-23}
}

@online{blogWithAuthor,
date = {2018},
title = {Historians in the News},
maintitle = {History Matters},
organization = {The University of Sydney},
url = {http://blogs.usyd.edu.au/historymatters/2018/02/historians_in_the_news_2018.html},
urldate = {2018-02-23},
author = {McDonnell, Mike}
}

usepackage{xpatch}% Modifies online bibdriver to remove author field, and add maintitle field after title
    xpatchbibdriver{online}
    {usebibmacro{author/editor+others/translator+others}%
    setunit{labelnamepunct}newblock
    usebibmacro{title}}
    {usebibmacro{date+extradate}%
    setunit{labelnamepunct}newblock
    usebibmacro{title}%
    newunitnewblock
    usebibmacro{maintitle}%
    newunit}
    {}
    {typeout{failed to remove author field, add maintitle field to driver for 'online'}}