Symptoms
No realm entries for 'http://blackpearlk2dev:81/Designer/' was found
Diagnoses
I found another article on the K2 forum: http://community.k2.com/t5/K2-blackpearl/No-realm-entries-were-found/ta-p/85439 and had a look in my identity tables.
The identity.claimrealm table had 6 entries
The identity.claimrealmissuer table only had 4 entries, going to RealmID 1,2,3,6
As per the article, I manually edited the claimrealmissuer table and added 4 and 5 to it. Importantly, 5 goes to http://blackpearlk2dev:81/Designer/ which is the designer URL
Resolution
The issue was resolved by ensuring all the values eK2].2Identity].yClaimRealm] are the same as the ones on the hK2].2Identity].yClaimRealmIssuer].
Please note that K2 does not support direct accesseditting of the database