LaTeX basically reads your document from beginning to end and can not skip forward when processing your file. In order to be able to act on stuff that happens later in your document, LaTeX remembers certain things from earlier LaTeX runs. That is the reason why for some things you need to run LaTeX multiple times to get things right. (For example for labels, see Understanding how references and labels work.) LaTeX remembers things from an earlier run by writing the relevant information into so-called auxiliary or temporary files. The most important auxiliary file is the
.aux file. The
.aux file is read quite early on when your document is processed and contains information from the previous LaTeX run.
The fact that the
.aux file was produced by the previous run becomes crucial here. When you load
biblatex, that package writes certain commands into the
.aux file to help typesetting the bibliography and citations as expected. Those commands are defined by
biblatex and will be executed on the next run to make sure everything looks nice.
If you now drop
biblatex and run LaTeX with an
.aux file produced by the previous run where
biblatex was still loaded, the
.aux file will contain references to
biblatex macros that are now unknown to LaTeX (you haven't loaded
biblatex, after all).
Errors like this can swiftly be resolved by deleting all temporary and auxiliary files (
.bcf, ...) and running a full compilation cycle from scratch. Deleting the auxiliary files is safe since they will be recreated in the next run of LaTeX (or the relevant helper program).