Create a user defined function thus:

Function blah(OrigStr As Range)
blah = ""
For i = 1 To Len(OrigStr)
If OrigStr.Characters(i, 1).Font.Bold = True Then blah = blah &
Mid(OrigStr, i, 1)
Next
End Function

(The line beginning
If...
ends with
Mid(OrigStr, i, 1)

It may be split above.)

Example of use in worksheet:
=blah(B3)