It's the old hymn, Welcome Voice. Popular in North America.

I hear thy tender voice
Calling me
To come and wash my sins
In the River of Calvary

Chorus:
Lord, here I am
At your call
Wash my spirit in the blood
That flows from Calvary.

Jesus invites me
To receive with the saints (gyda'i saint that should be)
Faith, hope, pure love and peace
And all of heaven's honours.

Chorus again

Praise be forever
For the Atonement and the Cleansing
I will receive Jesus (Iesu) as I am
And I will sing in the blood (ie. 'the Blood of the Lamb' is the usual English idiom)

However, I think most of us would sing, "A channaf yn y gwaed", which means, "I will be washed clean in the blood." Don't know if Cerys is mis-remembering or trying to modernize, but the original 'cannaf' (two 'n's) refers back to the first verse.

A lovely hymn and very much part of the Welsh 'psyche'!

siân