I had to configure the svn client on the target machine to access internet via a http proxy. I had to define values for the following properties in 'servers' file under '/root/.subversion' directory :
http-proxy-exceptions = [exceptions to proxy host here]
http-proxy-host = [value of proxy host]
http-proxy-port = 8080
http-compression = yes
After defining the above values, svn export command worked and I was able to export it using the external command inside a blpackage.