A good question,
The net gives number of different pronunciations e.g.

kah-eh-ru-LEH-a or see-ROO-lee-uh

The word is Latin meaning blue (opinions differ about the exact shade of blue, dark sky blue is one description) and in Latin a 'C' at the beginning of a word should be a hard 'C' so I would say (having studied Latin many years ago) that the correct pronunciation would be

K-er-u-lea.

Here is a link that might be useful: Passiflora caerulea

Interesting, as always Myles. I have always pronounced that shade of blue " suh - roo -lea.

I don't think that it is something to worry about either way. For example 'Caesar' most of us pronounce c-zar now but in Roman times it would have been ky-zar.
Myles