The only Somalis that have Arab blood are those from around Mogadishu and the many who are originally from Yemen. Before the war, there were a lot of Yemenis that lived in the south and they by and large are just like regular Somalis.
For the most part we are NOT mixed... in fact, due to the clan structures most Somalis are inbred since the xenophobia doesn't just extend to white folk, black folk, other Africans, Arabs... it occurs right in between the Somali community. That's why Somalia has been in civil war for 25 years at its root. Clan politics.
And yeah, Somalis are racist but some of it is also innocent racism like with East Asians we call them "small eyes" in Somali (Indha Yar).