Please, I add this code in my manuscript, but my citations are separated by ; instead of , (for example \cite{A1,A2} generates [A1; A2] but not [A1, A2]).
\documentclass{article}
\usepackage[style=alphabetic]{biblatex}
\addbibresource{biblatex-examples.bib}
\DeclareFieldFormat{labelalpha}{\mkbibbold{#1}}
\DeclareFieldFormat{extraalpha}{\mkbibbold{\mknumalph{#1}}}
\AtBeginBibliography{
\DeclareFieldFormat{labelalpha}{#1}
\DeclareFieldFormat{extraalpha}{\mknumalph{#1}}}
\begin{document}
Lorem \cite[43]{geer} ipsum.
\printbibliography
\end{document}
%at the end of lines in your\AtBeginBibliographyblock. (So\AtBeginBibliography{%and\DeclareFieldFormat{labelalpha}{#1}%) to avoid spurious spaces. – moewe Dec 16 '19 at 21:34biblatexversions expect biber, so if you use the default bibtex, the above code obviously won't work correctly. Either configure your editor to use biber or addbackend=bibtexto biblatex settings (so in your example it would be\usepackage[style=alphabetic,backend=bibtex]{biblatex}). – Disenchanted Lurker Dec 17 '19 at 13:03