After checking Yahoo discussions regarding blat and vcf files and after reading RequestID 785780 I succeeded in blat sending invitations to Lotus Notes users. Fine.
-- even with the -superdebugT option I do not really know why this works
-- it works only using the undocumented -penguin option
There shall be a possibility to enable the user to specify the correct type of his content.
either by adding new keywords like in the -enriched and -html options (in my case this would be -calendar)
or by adding a new option -contenttype enabling me to specify "-contenttype text/calendar"