This is a bit belated but we too saw the Common Grackle at Lake Balboa on Tues. 4/29, at about 11 AM. We couldn't find it on Monday 4/29 3 to 4 pm. We finally found it about 100-150 yds. south of the boat ramp after searching for it for about 20 minutes. It was on the lawn under the trees about halfway between the lake and the road. It didn't seem to be hanging out near the other icterids, but it didn't seem to be purposely avoiding them either.
Just in case you aren't sure what to look for: lovely adult plumage, glossy purple head with bright yellow eye, glossy somewhat bronzy body and tail, obvious vee-fold in tail, seems to be limping a bit but I couldn't figure out which foot it was favoring. A bit bigger than the Brewer's Blackbirds in the same area, but definitely smaller than the Great-tailed Grackles which are also common around the lake. Male GTGR's plumage is a nearly uniform glossy purple, and they're about 50% larger than the COGR. Female GTGR's are that saffron/ochre-brown from neck to belly, and they are also larger than the COGR.
I think the bird hangs out somewhere on the golf course when it's not at the lake.
Santa Monica, Ca