Main text of tcbtheorem environment absorbed into options

An environment defined with newtcbtheorem indeed requires two mandatory arguments:

begin{dfn}{}{}
Text of the definition
end{dfn}

The first argument is for the title, the second is for the label. With a call such as

begin{dfn}{Nice one}{}
Text of the definition
end{dfn}

the title would be set to “Nice one” and there would be no label. With

begin{dfn}{Nice one}{nice}
Text of the definition
end{dfn}

you can refer to the number of the definition by ref{dfn:nice} (the prefix before the colon is the last argument for newtcbtheorem.

You can also leave the title empty and set a label

begin{dfn}{}{nice}
Text of the definition
end{dfn}

That's the way it is.

---

You might define a wrapper around such theorems so you can use a different and, in my opinion, handier syntax.

documentclass[12pt]{report}
usepackage[most]{tcolorbox}
newtcbtheorem[number within = chapter]{dfninner}{Definition}%
 {
    enhanced,
    before title = {stepcounter{dfn}},
    colback=blue!10,
    colframe=blue!35!black,
    fonttitle=bfseries,
    top=3mm,
    attach boxed title to top left={xshift = 5mm, yshift=-1.5mm},
    boxed title style = {colback=blue!35!black}
}{dfn}
newcounter{dfn}
ExplSyntaxOn
NewDocumentEnvironment{dfn}{O{}}
 {
  keys_set:nn { hushus/tcb } { #1 }
  hushus_tcb_begin:nVV {dfninner} l__hushus_tcb_title_tl l__hushus_tcb_label_tl
 }
 {
  end{dfninner}
 }
cs_new_protected:Nn hushus_tcb_begin:nnn
 {
  begin{#1}{#2}{#3}
 }
cs_generate_variant:Nn hushus_tcb_begin:nnn { nVV }
keys_define:nn { hushus/tcb }
 {
  title .tl_set:N = l__hushus_tcb_title_tl,
  label .tl_set:N = l__hushus_tcb_label_tl,
 }
ExplSyntaxOff

begin{document}

begin{dfn}
    abc
end{dfn}

begin{dfn}[title=Nice,label=nice]
    abc
end{dfn}

ref{dfn:nice}

end{document}

An abstraction of the previous code that allows to use the standard tcolorbox syntax and takes care of the modifications for all environments you want to define.

documentclass[12pt]{report}
usepackage[most]{tcolorbox}

ExplSyntaxOn

NewDocumentCommand{betternewtcbtheorem}{O{}mmmm}
 {
  newtcbtheorem[#1]{#2inner}{#3}{#4}{#5}
  NewDocumentEnvironment{#2}{O{}}
   {
    keys_set:nn { hushus/tcb } { ##1 }
    hushus_tcb_begin:nVV {#2inner} l__hushus_tcb_title_tl l__hushus_tcb_label_tl
   }
   {
    end{#2inner}
   }
  cs_if_exist:cF { c@#5} { newcounter{#5} }
 }

cs_new_protected:Nn hushus_tcb_begin:nnn
 {
  begin{#1}{#2}{#3}
 }
cs_generate_variant:Nn hushus_tcb_begin:nnn { nVV }
keys_define:nn { hushus/tcb }
 {
  title .tl_set:N = l__hushus_tcb_title_tl,
  label .tl_set:N = l__hushus_tcb_label_tl,
 }

ExplSyntaxOff

betternewtcbtheorem[number within = chapter]{dfn}{Definition}%
 {
    enhanced,
    before title = {stepcounter{dfn}},
    colback=blue!10,
    colframe=blue!35!black,
    fonttitle=bfseries,
    top=3mm,
    attach boxed title to top left={xshift = 5mm, yshift=-1.5mm},
    boxed title style = {colback=blue!35!black}
}{dfn}


begin{document}

begin{dfn}
    abc
end{dfn}

begin{dfn}[title=Nice,label=nice]
    abc
end{dfn}

ref{dfn:nice}

end{document}

What's happening? The idea is that betternewtcbtheorem defines an “inner” environment that will be called by the one with the desired name.

In the example above, the desired name is dfn, so first we call newtcolorbox passing the arguments, but with inner appended to the name. Next the dfn environment is defined, taking an optional argument that should contain something with key-value syntax.

The allowed keys are title and label. When dfn is called, we pass the values in the proper place. Since these values are stored in token list variables, I define a function

hushus_tcb_begin:nnn

that will call begin{dfninner}{<title>}{<label>}, but I need to pass the contents of two variables, so this is the place for the variant

hushus_tcb_begin:nVV

that takes the first argument braced as usual, but the other two should be token list variables, whose content is passed to the main function as if they were braced arguments.

Final touch: a counter is defined if not previously existing, based on the last argument to newtcbtheorem.