count up to the next blank cell

The curly brackets tell Excel that this is a special Array formula... here is more on Array formulas that can explain better than I can:

http://www.cpearson.com/excel/ArrayFormulas.aspx

This formula:

=MATCH(TRUE;(INDEX($H$8:$H$5000;1):$H$5000="");0)

The INDEX($H$8:$H$5000;1) part just finds the cell reference indicated by the position within the array...i.e. position 1.... note: if it is always going to be position 1 (or H8), then you don't need the Index() part at all... you can just have
=MATCH(TRUE;$H$8:$H$5000="";0)

The H8:H5000="" part creates an array of TRUEs and FALSEs depending on if the condition is true or not....

Then the MATCH(TRUE()) looks for the first TRUE in that array and returns the position with the entire array.

If you select the cell with the formula and go to Tools|Formula Auditing|Evaluate Formula you can follow the evaluation of the formula by hitting Evaluate for each step. Then you will see how the result is obtained.

Hope this helps.