Suspect this can vary depending on the device you're using to access Blipfoto. With a browser on the likes of a PC, then:
If you want to add a link from a Blip's entry text, or from 'Your profile' in your journal's 'settings' area, the process is much the same. I find it easiest to highlight the word or words you want to use for the link, then click on the insert a link icon that's the right-most of the ones at the bottom of the text entry box (Bold; Italics; Underline; Strike-through; and Insert a Link).
A wee box should appear with a space for the URL.
I normally delete the "http://" that's already there, or paste over it with the URL you want. If you've highlighted some text, as mentioned above, there's no need to put anything in the space for, 'Description', just click the 'Insert' button. Don't forget to, 'Save changes'!
I hope that works for you.