sql server 2008 - where clause on parameter but if blank return all including null -

i'm extracting data on sql server 2008 r2 (and using script in microsoft excel query in end). query has number of where clauses based on parameters in excel cells.

however have issue script designing in management studio want parameter allowed leave blank. though if returns values not nulls. need along lines:

where .... , nd.title = case when @param1 = 'blank'                      "return including null"                      else "return" @param1 

but cannot work out how achieve this.

her how can this:

where .... , (nd.title = @param1 or @param1 = '' or @param1 null)