Another pic of the leaves

Looks like a cherry tree.

If it is a cherry, when should flowers start forming? I live in sunny southern CA

It will flower in March. Depending on rootstock and variety it might start flowering in year 2 to 5.

definitely looks like cherry. Looks almost exactly like my Black Gold tree.

It is a cherry. I wonder if it needs more chill hours than California can provide. Basically if the tree does not see cold winter weather then it might never fruit very well if at all.

Cristobalina is somewhat common in So. California. Could be Minnie Royal or Royal Lee. All 200 chills or less. Depends on where in SoCal you are (chill hours) and if the home owner knew what he was doing (or just liked a cherry tree's looks without the fruit). And how long the tree has been in the ground (at least 3 years from what I see but maybe not more). If your area has 400 or more then quite a few cultivars could have been tried.

Here is a link that might be useful: Cristobalina at Arboreum

You might need a pollinator for your cherry, if you want fruit next year, bing or stella will do the job.